[PATCH net-next] softirq: reduce latencies

[PATCH net-next] softirq: reduce latencies

[Eric Dumazet]

From: Eric Dumazet <edumazet@google.com>

In various network workloads, __do_softirq() latencies can be up
to 20 ms if HZ=1000, and 200 ms if HZ=100.

This is because we iterate 10 times in the softirq dispatcher,
and some actions can consume a lot of cycles.

This patch changes the fallback to ksoftirqd condition to :

- A time limit of 2 ms.
- need_resched() being set on current task

When one of this condition is met, we wakeup ksoftirqd for further
softirq processing if we still have pending softirqs.

I ran several benchmarks and got no significant difference in
throughput, but a very significant reduction of maximum latencies.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: David Miller <davem@davemloft.net>
Cc: Tom Herbert <therbert@google.com>
---
 kernel/softirq.c |   17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index ed567ba..64d61ea 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -195,21 +195,21 @@ void local_bh_enable_ip(unsigned long ip)
 EXPORT_SYMBOL(local_bh_enable_ip);
 
 /*
- * We restart softirq processing MAX_SOFTIRQ_RESTART times,
- * and we fall back to softirqd after that.
+ * We restart softirq processing for at most 2 ms,
+ * and if need_resched() is not set.
  *
- * This number has been established via experimentation.
+ * These limits have been established via experimentation.
  * The two things to balance is latency against fairness -
  * we want to handle softirqs as soon as possible, but they
  * should not be able to lock up the box.
  */
-#define MAX_SOFTIRQ_RESTART 10
+#define MAX_SOFTIRQ_TIME  min(1, (2*HZ/1000))
 
 asmlinkage void __do_softirq(void)
 {
 	struct softirq_action *h;
 	__u32 pending;
-	int max_restart = MAX_SOFTIRQ_RESTART;
+	unsigned long end = jiffies + MAX_SOFTIRQ_TIME;
 	int cpu;
 	unsigned long old_flags = current->flags;
 
@@ -264,11 +264,12 @@ restart:
 	local_irq_disable();
 
 	pending = local_softirq_pending();
-	if (pending && --max_restart)
-		goto restart;
+	if (pending) {
+		if (time_before(jiffies, end) && !need_resched())
+			goto restart;
 
-	if (pending)
 		wakeup_softirqd();
+	}
 
 	lockdep_softirq_exit();
 

Re: [PATCH net-next] softirq: reduce latencies

[Andrew Morton]

On Thu, 03 Jan 2013 04:28:52 -0800
Eric Dumazet <eric.dumazet@gmail.com> wrote:

> From: Eric Dumazet <edumazet@google.com>
> 
> In various network workloads, __do_softirq() latencies can be up
> to 20 ms if HZ=1000, and 200 ms if HZ=100.
> 
> This is because we iterate 10 times in the softirq dispatcher,
> and some actions can consume a lot of cycles.

hm, where did that "20 ms" come from?  What caused it?  Is it simply
the case that you happened to have actions which consume 2ms if HZ=1000
and 20ms if HZ=100?

> This patch changes the fallback to ksoftirqd condition to :
> 
> - A time limit of 2 ms.
> - need_resched() being set on current task
>
> When one of this condition is met, we wakeup ksoftirqd for further
> softirq processing if we still have pending softirqs.

Do we need both tests?  The need_resched() test alone might be
sufficient?


With this change, there is a possibility that a rapidly-rescheduling
task will cause softirq starvation?


Can this change cause worsened latencies in some situations?  Say there
are a large number of short-running actions queued.  Presently we'll
dispatch ten of them and return.  With this change we'll dispatch many
more of them - however many consume 2ms.  So worst-case latency
increases from "10 * not-much" to "2 ms".

Re: [PATCH net-next] softirq: reduce latencies

[Ben Hutchings]

On Thu, 2013-01-03 at 04:28 -0800, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@google.com>
> 
> In various network workloads, __do_softirq() latencies can be up
> to 20 ms if HZ=1000, and 200 ms if HZ=100.
> 
> This is because we iterate 10 times in the softirq dispatcher,
> and some actions can consume a lot of cycles.
> 
> This patch changes the fallback to ksoftirqd condition to :
> 
> - A time limit of 2 ms.
> - need_resched() being set on current task
[...]
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
[...]
> -#define MAX_SOFTIRQ_RESTART 10
> +#define MAX_SOFTIRQ_TIME  min(1, (2*HZ/1000))
[...]

Really?  Never iterate if HZ < 500?

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

Re: [PATCH net-next] softirq: reduce latencies

[Eric Dumazet]

On Thu, 2013-01-03 at 22:08 +0000, Ben Hutchings wrote:
> On Thu, 2013-01-03 at 04:28 -0800, Eric Dumazet wrote:
> > From: Eric Dumazet <edumazet@google.com>
> > 
> > In various network workloads, __do_softirq() latencies can be up
> > to 20 ms if HZ=1000, and 200 ms if HZ=100.
> > 
> > This is because we iterate 10 times in the softirq dispatcher,
> > and some actions can consume a lot of cycles.
> > 
> > This patch changes the fallback to ksoftirqd condition to :
> > 
> > - A time limit of 2 ms.
> > - need_resched() being set on current task
> [...]
> > --- a/kernel/softirq.c
> > +++ b/kernel/softirq.c
> [...]
> > -#define MAX_SOFTIRQ_RESTART 10
> > +#define MAX_SOFTIRQ_TIME  min(1, (2*HZ/1000))
> [...]
> 
> Really?  Never iterate if HZ < 500?
> 

good catch, it should be max()

Re: [PATCH net-next] softirq: reduce latencies

[Eric Dumazet]

On Thu, 2013-01-03 at 12:46 -0800, Andrew Morton wrote:
> On Thu, 03 Jan 2013 04:28:52 -0800
> Eric Dumazet <eric.dumazet@gmail.com> wrote:
> 
> > From: Eric Dumazet <edumazet@google.com>
> > 
> > In various network workloads, __do_softirq() latencies can be up
> > to 20 ms if HZ=1000, and 200 ms if HZ=100.
> > 
> > This is because we iterate 10 times in the softirq dispatcher,
> > and some actions can consume a lot of cycles.
> 
> hm, where did that "20 ms" come from?  What caused it?  Is it simply
> the case that you happened to have actions which consume 2ms if HZ=1000
> and 20ms if HZ=100?

net_rx_action() has such behavior yes.

In the worst/busy case, we spend 2 ticks per call, and even more
in some cases you dont want to know about (like triggering a IPv6 route
garbage collect)

> 
> > This patch changes the fallback to ksoftirqd condition to :
> > 
> > - A time limit of 2 ms.
> > - need_resched() being set on current task
> >
> > When one of this condition is met, we wakeup ksoftirqd for further
> > softirq processing if we still have pending softirqs.
> 
> Do we need both tests?  The need_resched() test alone might be
> sufficient?
> 

I tried a need_resched() only, but could trigger watchdog faults and
reboots, in case a cpu was dedicated to softirq and all other tasks run
on other cpus.

In other cases, the following RCU splat was triggered :

(need_resched() doesnt know the current cpu is blocking RCU )

Jan  2 21:33:40 lpq83 kernel: [  311.678050] INFO: rcu_sched self-detected stall on CPU { 2}  (t=21000 jiffies g=11416 c=11415 q=2665)
Jan  2 21:33:40 lpq83 kernel: [  311.687314] Pid: 1460, comm: simple-watchdog Not tainted 3.8.0-smp-DEV #63
Jan  2 21:33:40 lpq83 kernel: [  311.687316] Call Trace:
Jan  2 21:33:40 lpq83 kernel: [  311.687317]  <IRQ>  [<ffffffff81100e92>] rcu_check_callbacks+0x212/0x7a0
Jan  2 21:33:40 lpq83 kernel: [  311.687326]  [<ffffffff81097018>] update_process_times+0x48/0x90
Jan  2 21:33:40 lpq83 kernel: [  311.687329]  [<ffffffff810cfe31>] tick_sched_timer+0x81/0xd0
Jan  2 21:33:40 lpq83 kernel: [  311.687332]  [<ffffffff810ad69d>] __run_hrtimer+0x7d/0x220
Jan  2 21:33:40 lpq83 kernel: [  311.687333]  [<ffffffff810cfdb0>] ? tick_nohz_handler+0x100/0x100
Jan  2 21:33:40 lpq83 kernel: [  311.687337]  [<ffffffff810ca02c>] ? ktime_get_update_offsets+0x4c/0xd0
Jan  2 21:33:40 lpq83 kernel: [  311.687339]  [<ffffffff810adfa7>] hrtimer_interrupt+0xf7/0x230
Jan  2 21:33:40 lpq83 kernel: [  311.687343]  [<ffffffff815b7089>] smp_apic_timer_interrupt+0x69/0x99
Jan  2 21:33:40 lpq83 kernel: [  311.687345]  [<ffffffff815b630a>] apic_timer_interrupt+0x6a/0x70
Jan  2 21:33:40 lpq83 kernel: [  311.687348]  [<ffffffffa01a1b66>] ? ipt_do_table+0x106/0x5b0 [ip_tables]
Jan  2 21:33:40 lpq83 kernel: [  311.687352]  [<ffffffff810fb357>] ? handle_edge_irq+0x77/0x130
Jan  2 21:33:40 lpq83 kernel: [  311.687354]  [<ffffffff8108e9c9>] ? irq_exit+0x79/0xb0
Jan  2 21:33:40 lpq83 kernel: [  311.687356]  [<ffffffff815b6fa3>] ? do_IRQ+0x63/0xe0
Jan  2 21:33:40 lpq83 kernel: [  311.687359]  [<ffffffffa004a0d3>] iptable_filter_hook+0x33/0x64 [iptable_filter]
Jan  2 21:33:40 lpq83 kernel: [  311.687362]  [<ffffffff81523dff>] nf_iterate+0x8f/0xd0
Jan  2 21:33:40 lpq83 kernel: [  311.687364]  [<ffffffff81529fc0>] ? ip_rcv_finish+0x360/0x360
Jan  2 21:33:40 lpq83 kernel: [  311.687366]  [<ffffffff81523ebd>] nf_hook_slow+0x7d/0x150
Jan  2 21:33:40 lpq83 kernel: [  311.687368]  [<ffffffff81529fc0>] ? ip_rcv_finish+0x360/0x360
Jan  2 21:33:40 lpq83 kernel: [  311.687370]  [<ffffffff8152a39e>] ip_local_deliver+0x5e/0xa0
Jan  2 21:33:40 lpq83 kernel: [  311.687372]  [<ffffffff81529d79>] ip_rcv_finish+0x119/0x360
Jan  2 21:33:40 lpq83 kernel: [  311.687374]  [<ffffffff8152a631>] ip_rcv+0x251/0x300
Jan  2 21:33:40 lpq83 kernel: [  311.687377]  [<ffffffff814f4c72>] __netif_receive_skb+0x582/0x820
Jan  2 21:33:40 lpq83 kernel: [  311.687379]  [<ffffffff81560697>] ? inet_gro_receive+0x197/0x200
Jan  2 21:33:40 lpq83 kernel: [  311.687381]  [<ffffffff814f50ad>] netif_receive_skb+0x2d/0x90
Jan  2 21:33:40 lpq83 kernel: [  311.687383]  [<ffffffff814f5943>] napi_gro_frags+0xf3/0x2a0
Jan  2 21:33:40 lpq83 kernel: [  311.687387]  [<ffffffffa01aa87c>] mlx4_en_process_rx_cq+0x6cc/0x7b0 [mlx4_en]
Jan  2 21:33:40 lpq83 kernel: [  311.687390]  [<ffffffffa01aa9ff>] mlx4_en_poll_rx_cq+0x3f/0x80 [mlx4_en]
Jan  2 21:33:40 lpq83 kernel: [  311.687392]  [<ffffffff814f53c1>] net_rx_action+0x111/0x210
Jan  2 21:33:40 lpq83 kernel: [  311.687393]  [<ffffffff814f3a51>] ? net_tx_action+0x81/0x1d0


> 
> With this change, there is a possibility that a rapidly-rescheduling
> task will cause softirq starvation?
> 

Only if this task has higher priority than ksoftirqd, but then, people
wanting/playing with high priority tasks know what they do ;)

> 
> Can this change cause worsened latencies in some situations?  Say there
> are a large number of short-running actions queued.  Presently we'll
> dispatch ten of them and return.  With this change we'll dispatch many
> more of them - however many consume 2ms.  So worst-case latency
> increases from "10 * not-much" to "2 ms".

I tried to reproduce such workload but couldnt. 2 ms (or more exactly 1
to 2 ms given the jiffies/HZ granularity) is about the time needed to
process 1000 frames on current hardware.

Certainly, this patch will increase number of scheduler calls in some
situations. But with the increase of cores, it seems a bit odd to allow
softirq to be a bad guy. The current logic was more suited for the !SMP
age.

Re: [PATCH net-next] softirq: reduce latencies

[Namhyung Kim]

Hi,

On Thu, 03 Jan 2013 14:41:15 -0800, Eric Dumazet wrote:
> On Thu, 2013-01-03 at 12:46 -0800, Andrew Morton wrote:
>> Can this change cause worsened latencies in some situations?  Say there
>> are a large number of short-running actions queued.  Presently we'll
>> dispatch ten of them and return.  With this change we'll dispatch many
>> more of them - however many consume 2ms.  So worst-case latency
>> increases from "10 * not-much" to "2 ms".
>
> I tried to reproduce such workload but couldnt. 2 ms (or more exactly 1
> to 2 ms given the jiffies/HZ granularity) is about the time needed to
> process 1000 frames on current hardware.

Probably a silly question:

Why not using ktime rather than jiffies for this?

Thanks,
Namhyung

Re: [PATCH net-next] softirq: reduce latencies

[Eric Dumazet]

On Fri, 2013-01-04 at 14:16 +0900, Namhyung Kim wrote:

> Probably a silly question:
> 
> Why not using ktime rather than jiffies for this?

ktime is too expensive on some hardware.

Here we only want a safety belt, no need for high time resolution.

Re: [PATCH net-next] softirq: reduce latencies

[Eric Dumazet]

On Fri, 2013-01-04 at 11:14 +0400, Oleg A.Arkhangelsky wrote:

> It leads to many context switches when softirqs processing deffered to
> ksoftirqd kthreads which can be very expensive. Here is some evidence
> of ksoftirqd activation effects:
> 
> http://marc.info/?l=linux-netdev&m=124116262916969&w=2
> 
> Look for "magic threshold". Yes, I know there was another bug in scheduler
> discovered that time, but this bug was only about tick accounting.
> 

This thread is 3 years old : 

- It was a router workload. Forwarded packets should not wakeup a task.
- The measure of how cpus spent their cycles was completely wrong.
- A lot of things have changed, both in network stack and scheduler.

In fact, under moderate load, my patch is able to loop more than 10
times before deferring to ksoftirqd.

Under stress, ksoftirqd will be started anyway, and its a good thing,
because it enables process migration.

500 "context switches" [1] per second instead of 50 on behalf of
ksoftirqd is absolutely not measurable. It also permits smoother RCU
cleanups.

I did a lot of benchmarks, and didnt see any regression yet, but usual
noise.

[1] Under load, __do_softirq() would be called 500 times per second,
instead of ~50 times per second.

[PATCH v2 net-next] softirq: reduce latencies

[Eric Dumazet]

From: Eric Dumazet <edumazet@google.com>

In various network workloads, __do_softirq() latencies can be up
to 20 ms if HZ=1000, and 200 ms if HZ=100.

This is because we iterate 10 times in the softirq dispatcher,
and some actions can consume a lot of cycles.

This patch changes the fallback to ksoftirqd condition to :

- A time limit of 2 ms.
- need_resched() being set on current task

When one of this condition is met, we wakeup ksoftirqd for further
softirq processing if we still have pending softirqs.

Using need_resched() as the only condition can trigger RCU stalls,
as we can keep BH disabled for too long.

I ran several benchmarks and got no significant difference in
throughput, but a very significant reduction of latencies (one order
of magnitude) :

In following bench, 200 antagonist "netperf -t TCP_RR" are started in
background, using all available cpus.

Then we start one "netperf -t TCP_RR", bound to the cpu handling the NIC
IRQ (hard+soft)

Before patch :

# netperf -H 7.7.7.84 -t TCP_RR -T2,2 -- -k
RT_LATENCY,MIN_LATENCY,MAX_LATENCY,P50_LATENCY,P90_LATENCY,P99_LATENCY,MEAN_LATENCY,STDDEV_LATENCY
MIGRATED TCP REQUEST/RESPONSE TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET
to 7.7.7.84 () port 0 AF_INET : first burst 0 : cpu bind
RT_LATENCY=550110.424
MIN_LATENCY=146858
MAX_LATENCY=997109
P50_LATENCY=305000
P90_LATENCY=550000
P99_LATENCY=710000
MEAN_LATENCY=376989.12
STDDEV_LATENCY=184046.92

After patch :

# netperf -H 7.7.7.84 -t TCP_RR -T2,2 -- -k
RT_LATENCY,MIN_LATENCY,MAX_LATENCY,P50_LATENCY,P90_LATENCY,P99_LATENCY,MEAN_LATENCY,STDDEV_LATENCY
MIGRATED TCP REQUEST/RESPONSE TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET
to 7.7.7.84 () port 0 AF_INET : first burst 0 : cpu bind
RT_LATENCY=40545.492
MIN_LATENCY=9834
MAX_LATENCY=78366
P50_LATENCY=33583
P90_LATENCY=59000
P99_LATENCY=69000
MEAN_LATENCY=38364.67
STDDEV_LATENCY=12865.26

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: David Miller <davem@davemloft.net>
Cc: Tom Herbert <therbert@google.com>
Cc: Ben Hutchings <bhutchings@solarflare.com>
---
v2: min(1, (2*HZ/1000)) -> max(1, (2*HZ/1000)), as spotted by Ben

 kernel/softirq.c |   17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index ed567ba..8d5e4be 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -195,21 +195,21 @@ void local_bh_enable_ip(unsigned long ip)
 EXPORT_SYMBOL(local_bh_enable_ip);
 
 /*
- * We restart softirq processing MAX_SOFTIRQ_RESTART times,
- * and we fall back to softirqd after that.
+ * We restart softirq processing for at most 2 ms,
+ * and if need_resched() is not set.
  *
- * This number has been established via experimentation.
+ * These limits have been established via experimentation.
  * The two things to balance is latency against fairness -
  * we want to handle softirqs as soon as possible, but they
  * should not be able to lock up the box.
  */
-#define MAX_SOFTIRQ_RESTART 10
+#define MAX_SOFTIRQ_TIME  max(1, (2*HZ/1000))
 
 asmlinkage void __do_softirq(void)
 {
 	struct softirq_action *h;
 	__u32 pending;
-	int max_restart = MAX_SOFTIRQ_RESTART;
+	unsigned long end = jiffies + MAX_SOFTIRQ_TIME;
 	int cpu;
 	unsigned long old_flags = current->flags;
 
@@ -264,11 +264,12 @@ restart:
 	local_irq_disable();
 
 	pending = local_softirq_pending();
-	if (pending && --max_restart)
-		goto restart;
+	if (pending) {
+		if (time_before(jiffies, end) && !need_resched())
+			goto restart;
 
-	if (pending)
 		wakeup_softirqd();
+	}
 
 	lockdep_softirq_exit();
 

Re: [PATCH v2 net-next] softirq: reduce latencies

[Joe Perches]

On Thu, 2013-01-03 at 23:49 -0800, Eric Dumazet wrote:
> In various network workloads, __do_softirq() latencies can be up
> to 20 ms if HZ=1000, and 200 ms if HZ=100.
> This patch changes the fallback to ksoftirqd condition to :
> - A time limit of 2 ms.

[]
> diff --git a/kernel/softirq.c b/kernel/softirq.c
[]
> +#define MAX_SOFTIRQ_TIME  max(1, (2*HZ/1000))

And if HZ is 10000?
 
>  asmlinkage void __do_softirq(void)
>  {
[]
> +	unsigned long end = jiffies + MAX_SOFTIRQ_TIME;

Perhaps MAX_SOFTIRQ_TIME should be

#define MAX_SOFTIRQ_TIME msecs_to_jiffies(2)

though it would be nicer if it were a compile time constant.

Re: [PATCH v2 net-next] softirq: reduce latencies

[Eric Dumazet]

On Fri, 2013-01-04 at 00:15 -0800, Joe Perches wrote:
> On Thu, 2013-01-03 at 23:49 -0800, Eric Dumazet wrote:
> > In various network workloads, __do_softirq() latencies can be up
> > to 20 ms if HZ=1000, and 200 ms if HZ=100.
> > This patch changes the fallback to ksoftirqd condition to :
> > - A time limit of 2 ms.
> 
> []
> > diff --git a/kernel/softirq.c b/kernel/softirq.c
> []
> > +#define MAX_SOFTIRQ_TIME  max(1, (2*HZ/1000))
> 
> And if HZ is 10000?
>  

Then its OK.  2*10000/1000 -> 20 ticks -> 2 ms


> >  asmlinkage void __do_softirq(void)
> >  {
> []
> > +	unsigned long end = jiffies + MAX_SOFTIRQ_TIME;
> 
> Perhaps MAX_SOFTIRQ_TIME should be
> 
> #define MAX_SOFTIRQ_TIME msecs_to_jiffies(2)
> 
> though it would be nicer if it were a compile time constant.

If you send a patch to convert msecs_to_jiffies() to an inline function
when HZ = 1000, I will gladly use it instead of (2*HZ/1000)

Right now, max(1, msecs_to_jiffies(2)) uses way too many instructions,
while it should be the constant 2, known at compile time.

Re: [PATCH v2 net-next] softirq: reduce latencies

[Joe Perches]

On Fri, 2013-01-04 at 00:23 -0800, Eric Dumazet wrote:
> On Fri, 2013-01-04 at 00:15 -0800, Joe Perches wrote:
> > Perhaps MAX_SOFTIRQ_TIME should be
> > #define MAX_SOFTIRQ_TIME msecs_to_jiffies(2)
> > though it would be nicer if it were a compile time constant.
> 
> If you send a patch to convert msecs_to_jiffies() to an inline function
> when HZ = 1000, I will gladly use it instead of (2*HZ/1000)
> 
> Right now, max(1, msecs_to_jiffies(2)) uses way too many instructions,
> while it should be the constant 2, known at compile time.

Something like this might work.

This is incomplete, it just does msecs_to_jiffies,
and it should convert usecs_to_jiffies and the
jiffies_to_<foo> types too.

Maybe it's worthwhile.

It does reduce object size by 16 bytes per call site
(x86-32) when the argument is a constant. There are
about 800 of these jiffies conversions in kernel sources.

What do you think?

 include/linux/jiffies.h | 49 ++++++++++++++++++++++++++++++++++++++++++++++++-
 kernel/time.c           | 42 +++---------------------------------------
 2 files changed, 51 insertions(+), 40 deletions(-)

diff --git a/include/linux/jiffies.h b/include/linux/jiffies.h
index 82ed068..c67ddcf 100644
--- a/include/linux/jiffies.h
+++ b/include/linux/jiffies.h
@@ -291,7 +291,54 @@ extern unsigned long preset_lpj;
  */
 extern unsigned int jiffies_to_msecs(const unsigned long j);
 extern unsigned int jiffies_to_usecs(const unsigned long j);
-extern unsigned long msecs_to_jiffies(const unsigned int m);
+extern unsigned long __msecs_to_jiffies(const unsigned int m);
+
+static inline unsigned long __inline_msecs_to_jiffies(const unsigned int m)
+{
+	/*
+	 * Negative value, means infinite timeout:
+	 */
+	if ((int)m < 0)
+		return MAX_JIFFY_OFFSET;
+
+#if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ)
+	/*
+	 * HZ is equal to or smaller than 1000, and 1000 is a nice
+	 * round multiple of HZ, divide with the factor between them,
+	 * but round upwards:
+	 */
+	return (m + (MSEC_PER_SEC / HZ) - 1) / (MSEC_PER_SEC / HZ);
+#elif HZ > MSEC_PER_SEC && !(HZ % MSEC_PER_SEC)
+	/*
+	 * HZ is larger than 1000, and HZ is a nice round multiple of
+	 * 1000 - simply multiply with the factor between them.
+	 *
+	 * But first make sure the multiplication result cannot
+	 * overflow:
+	 */
+	if (m > jiffies_to_msecs(MAX_JIFFY_OFFSET))
+		return MAX_JIFFY_OFFSET;
+
+	return m * (HZ / MSEC_PER_SEC);
+#else
+	/*
+	 * Generic case - multiply, round and divide. But first
+	 * check that if we are doing a net multiplication, that
+	 * we wouldn't overflow:
+	 */
+	if (HZ > MSEC_PER_SEC && m > jiffies_to_msecs(MAX_JIFFY_OFFSET))
+		return MAX_JIFFY_OFFSET;
+
+	return (MSEC_TO_HZ_MUL32 * m + MSEC_TO_HZ_ADJ32)
+		>> MSEC_TO_HZ_SHR32;
+#endif
+}
+
+#define msecs_to_jiffies(x)			\
+	(__builtin_constant_p(x) ?		\
+	 __inline_msecs_to_jiffies(x) :		\
+	 __msecs_to_jiffies(x))
+
 extern unsigned long usecs_to_jiffies(const unsigned int u);
 extern unsigned long timespec_to_jiffies(const struct timespec *value);
 extern void jiffies_to_timespec(const unsigned long jiffies,
diff --git a/kernel/time.c b/kernel/time.c
index d226c6a..231f2ac 100644
--- a/kernel/time.c
+++ b/kernel/time.c
@@ -425,47 +425,11 @@ EXPORT_SYMBOL(ns_to_timeval);
  *
  * We must also be careful about 32-bit overflows.
  */
-unsigned long msecs_to_jiffies(const unsigned int m)
+unsigned long __msecs_to_jiffies(const unsigned int m)
 {
-	/*
-	 * Negative value, means infinite timeout:
-	 */
-	if ((int)m < 0)
-		return MAX_JIFFY_OFFSET;
-
-#if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ)
-	/*
-	 * HZ is equal to or smaller than 1000, and 1000 is a nice
-	 * round multiple of HZ, divide with the factor between them,
-	 * but round upwards:
-	 */
-	return (m + (MSEC_PER_SEC / HZ) - 1) / (MSEC_PER_SEC / HZ);
-#elif HZ > MSEC_PER_SEC && !(HZ % MSEC_PER_SEC)
-	/*
-	 * HZ is larger than 1000, and HZ is a nice round multiple of
-	 * 1000 - simply multiply with the factor between them.
-	 *
-	 * But first make sure the multiplication result cannot
-	 * overflow:
-	 */
-	if (m > jiffies_to_msecs(MAX_JIFFY_OFFSET))
-		return MAX_JIFFY_OFFSET;
-
-	return m * (HZ / MSEC_PER_SEC);
-#else
-	/*
-	 * Generic case - multiply, round and divide. But first
-	 * check that if we are doing a net multiplication, that
-	 * we wouldn't overflow:
-	 */
-	if (HZ > MSEC_PER_SEC && m > jiffies_to_msecs(MAX_JIFFY_OFFSET))
-		return MAX_JIFFY_OFFSET;
-
-	return (MSEC_TO_HZ_MUL32 * m + MSEC_TO_HZ_ADJ32)
-		>> MSEC_TO_HZ_SHR32;
-#endif
+	return __inline_msecs_to_jiffies(m);
 }
-EXPORT_SYMBOL(msecs_to_jiffies);
+EXPORT_SYMBOL(__msecs_to_jiffies);
 
 unsigned long usecs_to_jiffies(const unsigned int u)
 {

Re: [PATCH v2 net-next] softirq: reduce latencies

[Eric Dumazet]

On Fri, 2013-01-04 at 01:12 -0800, Joe Perches wrote:
> On Fri, 2013-01-04 at 00:23 -0800, Eric Dumazet wrote:
> > On Fri, 2013-01-04 at 00:15 -0800, Joe Perches wrote:
> > > Perhaps MAX_SOFTIRQ_TIME should be
> > > #define MAX_SOFTIRQ_TIME msecs_to_jiffies(2)
> > > though it would be nicer if it were a compile time constant.
> > 
> > If you send a patch to convert msecs_to_jiffies() to an inline function
> > when HZ = 1000, I will gladly use it instead of (2*HZ/1000)
> > 
> > Right now, max(1, msecs_to_jiffies(2)) uses way too many instructions,
> > while it should be the constant 2, known at compile time.
> 
> Something like this might work.
> 
> This is incomplete, it just does msecs_to_jiffies,
> and it should convert usecs_to_jiffies and the
> jiffies_to_<foo> types too.
> 
> Maybe it's worthwhile.
> 
> It does reduce object size by 16 bytes per call site
> (x86-32) when the argument is a constant. There are
> about 800 of these jiffies conversions in kernel sources.
> 
> What do you think?
> 

I think this is something to discuss in another thread, and definitely
worth to do, at least for msecs_to_jiffies()

We have many HZ references everywhere that could be cleaned up using
this.

[PATCH] jiffies conversions: Use compile time constants when possible

[Joe Perches]

Do the multiplications and divisions at compile time
instead of runtime when the converted value is a constant.

Make the calculation functions static __always_inline to jiffies.h.

Add #defines with __builtin_constant_p to test and use the
static inline or the runtime functions as appropriate.

Prefix the old exported symbols/functions with __

Signed-off-by: Joe Perches <joe@perches.com>
---
 include/linux/jiffies.h | 136 ++++++++++++++++++++++++++++++++++++++++++++++--
 kernel/time.c           | 106 +++++--------------------------------
 2 files changed, 144 insertions(+), 98 deletions(-)

diff --git a/include/linux/jiffies.h b/include/linux/jiffies.h
index 82ed068..88578e4 100644
--- a/include/linux/jiffies.h
+++ b/include/linux/jiffies.h
@@ -289,10 +289,138 @@ extern unsigned long preset_lpj;
 /*
  * Convert various time units to each other:
  */
-extern unsigned int jiffies_to_msecs(const unsigned long j);
-extern unsigned int jiffies_to_usecs(const unsigned long j);
-extern unsigned long msecs_to_jiffies(const unsigned int m);
-extern unsigned long usecs_to_jiffies(const unsigned int u);
+
+/*
+ * Avoid unnecessary multiplications/divisions in the
+ * two most common HZ cases:
+ */
+static __always_inline unsigned int
+__inline_jiffies_to_msecs(const unsigned long j)
+{
+#if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ)
+	return (MSEC_PER_SEC / HZ) * j;
+#elif HZ > MSEC_PER_SEC && !(HZ % MSEC_PER_SEC)
+	return (j + (HZ / MSEC_PER_SEC) - 1)/(HZ / MSEC_PER_SEC);
+#else
+# if BITS_PER_LONG == 32
+	return (HZ_TO_MSEC_MUL32 * j) >> HZ_TO_MSEC_SHR32;
+# else
+	return (j * HZ_TO_MSEC_NUM) / HZ_TO_MSEC_DEN;
+# endif
+#endif
+}
+extern unsigned int __jiffies_to_msecs(const unsigned long j);
+
+#define jiffies_to_msecs(x)			\
+	(__builtin_constant_p(x) ?		\
+	 __inline_jiffies_to_msecs(x) :		\
+	 __jiffies_to_msecs(x))
+
+static __always_inline unsigned int
+__inline_jiffies_to_usecs(const unsigned long j)
+{
+#if HZ <= USEC_PER_SEC && !(USEC_PER_SEC % HZ)
+	return (USEC_PER_SEC / HZ) * j;
+#elif HZ > USEC_PER_SEC && !(HZ % USEC_PER_SEC)
+	return (j + (HZ / USEC_PER_SEC) - 1)/(HZ / USEC_PER_SEC);
+#else
+# if BITS_PER_LONG == 32
+	return (HZ_TO_USEC_MUL32 * j) >> HZ_TO_USEC_SHR32;
+# else
+	return (j * HZ_TO_USEC_NUM) / HZ_TO_USEC_DEN;
+# endif
+#endif
+}
+extern unsigned int __jiffies_to_usecs(const unsigned long j);
+
+#define jiffies_to_usecs(x)			\
+	(__builtin_constant_p(x) ?		\
+	 __inline_jiffies_to_usecs(x) :		\
+	 __jiffies_to_usecs(x))
+
+/*
+ * When we convert to jiffies then we interpret incoming values
+ * the following way:
+ *
+ * - negative values mean 'infinite timeout' (MAX_JIFFY_OFFSET)
+ *
+ * - 'too large' values [that would result in larger than
+ *   MAX_JIFFY_OFFSET values] mean 'infinite timeout' too.
+ *
+ * - all other values are converted to jiffies by either multiplying
+ *   the input value by a factor or dividing it with a factor
+ *
+ * We must also be careful about 32-bit overflows.
+ */
+static __always_inline unsigned long
+__inline_msecs_to_jiffies(const unsigned int m)
+{
+	/*
+	 * Negative value, means infinite timeout:
+	 */
+	if ((int)m < 0)
+		return MAX_JIFFY_OFFSET;
+
+#if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ)
+	/*
+	 * HZ is equal to or smaller than 1000, and 1000 is a nice
+	 * round multiple of HZ, divide with the factor between them,
+	 * but round upwards:
+	 */
+	return (m + (MSEC_PER_SEC / HZ) - 1) / (MSEC_PER_SEC / HZ);
+#elif HZ > MSEC_PER_SEC && !(HZ % MSEC_PER_SEC)
+	/*
+	 * HZ is larger than 1000, and HZ is a nice round multiple of
+	 * 1000 - simply multiply with the factor between them.
+	 *
+	 * But first make sure the multiplication result cannot
+	 * overflow:
+	 */
+	if (m > jiffies_to_msecs(MAX_JIFFY_OFFSET))
+		return MAX_JIFFY_OFFSET;
+
+	return m * (HZ / MSEC_PER_SEC);
+#else
+	/*
+	 * Generic case - multiply, round and divide. But first
+	 * check that if we are doing a net multiplication, that
+	 * we wouldn't overflow:
+	 */
+	if (HZ > MSEC_PER_SEC && m > jiffies_to_msecs(MAX_JIFFY_OFFSET))
+		return MAX_JIFFY_OFFSET;
+
+	return (MSEC_TO_HZ_MUL32 * m + MSEC_TO_HZ_ADJ32)
+		>> MSEC_TO_HZ_SHR32;
+#endif
+}
+extern unsigned long __msecs_to_jiffies(const unsigned int m);
+
+#define msecs_to_jiffies(x)			\
+	(__builtin_constant_p(x) ?		\
+	 __inline_msecs_to_jiffies(x) :		\
+	 __msecs_to_jiffies(x))
+
+static __always_inline unsigned long
+__inline_usecs_to_jiffies(const unsigned int u)
+{
+	if (u > jiffies_to_usecs(MAX_JIFFY_OFFSET))
+		return MAX_JIFFY_OFFSET;
+#if HZ <= USEC_PER_SEC && !(USEC_PER_SEC % HZ)
+	return (u + (USEC_PER_SEC / HZ) - 1) / (USEC_PER_SEC / HZ);
+#elif HZ > USEC_PER_SEC && !(HZ % USEC_PER_SEC)
+	return u * (HZ / USEC_PER_SEC);
+#else
+	return (USEC_TO_HZ_MUL32 * u + USEC_TO_HZ_ADJ32)
+		>> USEC_TO_HZ_SHR32;
+#endif
+}
+extern unsigned long __usecs_to_jiffies(const unsigned int u);
+
+#define usecs_to_jiffies(x)			\
+	(__builtin_constant_p(x) ?		\
+	 __inline_usecs_to_jiffies(x) :		\
+	 __usecs_to_jiffies(x))
+
 extern unsigned long timespec_to_jiffies(const struct timespec *value);
 extern void jiffies_to_timespec(const unsigned long jiffies,
 				struct timespec *value);
diff --git a/kernel/time.c b/kernel/time.c
index d226c6a..b9d1024 100644
--- a/kernel/time.c
+++ b/kernel/time.c
@@ -228,41 +228,18 @@ EXPORT_SYMBOL(current_fs_time);
 
 /*
  * Convert jiffies to milliseconds and back.
- *
- * Avoid unnecessary multiplications/divisions in the
- * two most common HZ cases:
  */
-inline unsigned int jiffies_to_msecs(const unsigned long j)
+unsigned int __jiffies_to_msecs(const unsigned long j)
 {
-#if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ)
-	return (MSEC_PER_SEC / HZ) * j;
-#elif HZ > MSEC_PER_SEC && !(HZ % MSEC_PER_SEC)
-	return (j + (HZ / MSEC_PER_SEC) - 1)/(HZ / MSEC_PER_SEC);
-#else
-# if BITS_PER_LONG == 32
-	return (HZ_TO_MSEC_MUL32 * j) >> HZ_TO_MSEC_SHR32;
-# else
-	return (j * HZ_TO_MSEC_NUM) / HZ_TO_MSEC_DEN;
-# endif
-#endif
+	return __inline_jiffies_to_msecs(j);
 }
-EXPORT_SYMBOL(jiffies_to_msecs);
+EXPORT_SYMBOL(__jiffies_to_msecs);
 
-inline unsigned int jiffies_to_usecs(const unsigned long j)
+unsigned int __jiffies_to_usecs(const unsigned long j)
 {
-#if HZ <= USEC_PER_SEC && !(USEC_PER_SEC % HZ)
-	return (USEC_PER_SEC / HZ) * j;
-#elif HZ > USEC_PER_SEC && !(HZ % USEC_PER_SEC)
-	return (j + (HZ / USEC_PER_SEC) - 1)/(HZ / USEC_PER_SEC);
-#else
-# if BITS_PER_LONG == 32
-	return (HZ_TO_USEC_MUL32 * j) >> HZ_TO_USEC_SHR32;
-# else
-	return (j * HZ_TO_USEC_NUM) / HZ_TO_USEC_DEN;
-# endif
-#endif
+	return __inline_jiffies_to_usecs(j);
 }
-EXPORT_SYMBOL(jiffies_to_usecs);
+EXPORT_SYMBOL(__jiffies_to_usecs);
 
 /**
  * timespec_trunc - Truncate timespec to a granularity
@@ -411,76 +388,17 @@ struct timeval ns_to_timeval(const s64 nsec)
 }
 EXPORT_SYMBOL(ns_to_timeval);
 
-/*
- * When we convert to jiffies then we interpret incoming values
- * the following way:
- *
- * - negative values mean 'infinite timeout' (MAX_JIFFY_OFFSET)
- *
- * - 'too large' values [that would result in larger than
- *   MAX_JIFFY_OFFSET values] mean 'infinite timeout' too.
- *
- * - all other values are converted to jiffies by either multiplying
- *   the input value by a factor or dividing it with a factor
- *
- * We must also be careful about 32-bit overflows.
- */
-unsigned long msecs_to_jiffies(const unsigned int m)
+unsigned long __msecs_to_jiffies(const unsigned int m)
 {
-	/*
-	 * Negative value, means infinite timeout:
-	 */
-	if ((int)m < 0)
-		return MAX_JIFFY_OFFSET;
-
-#if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ)
-	/*
-	 * HZ is equal to or smaller than 1000, and 1000 is a nice
-	 * round multiple of HZ, divide with the factor between them,
-	 * but round upwards:
-	 */
-	return (m + (MSEC_PER_SEC / HZ) - 1) / (MSEC_PER_SEC / HZ);
-#elif HZ > MSEC_PER_SEC && !(HZ % MSEC_PER_SEC)
-	/*
-	 * HZ is larger than 1000, and HZ is a nice round multiple of
-	 * 1000 - simply multiply with the factor between them.
-	 *
-	 * But first make sure the multiplication result cannot
-	 * overflow:
-	 */
-	if (m > jiffies_to_msecs(MAX_JIFFY_OFFSET))
-		return MAX_JIFFY_OFFSET;
-
-	return m * (HZ / MSEC_PER_SEC);
-#else
-	/*
-	 * Generic case - multiply, round and divide. But first
-	 * check that if we are doing a net multiplication, that
-	 * we wouldn't overflow:
-	 */
-	if (HZ > MSEC_PER_SEC && m > jiffies_to_msecs(MAX_JIFFY_OFFSET))
-		return MAX_JIFFY_OFFSET;
-
-	return (MSEC_TO_HZ_MUL32 * m + MSEC_TO_HZ_ADJ32)
-		>> MSEC_TO_HZ_SHR32;
-#endif
+	return __inline_msecs_to_jiffies(m);
 }
-EXPORT_SYMBOL(msecs_to_jiffies);
+EXPORT_SYMBOL(__msecs_to_jiffies);
 
-unsigned long usecs_to_jiffies(const unsigned int u)
+unsigned long __usecs_to_jiffies(const unsigned int u)
 {
-	if (u > jiffies_to_usecs(MAX_JIFFY_OFFSET))
-		return MAX_JIFFY_OFFSET;
-#if HZ <= USEC_PER_SEC && !(USEC_PER_SEC % HZ)
-	return (u + (USEC_PER_SEC / HZ) - 1) / (USEC_PER_SEC / HZ);
-#elif HZ > USEC_PER_SEC && !(HZ % USEC_PER_SEC)
-	return u * (HZ / USEC_PER_SEC);
-#else
-	return (USEC_TO_HZ_MUL32 * u + USEC_TO_HZ_ADJ32)
-		>> USEC_TO_HZ_SHR32;
-#endif
+	return __inline_usecs_to_jiffies(u);
 }
-EXPORT_SYMBOL(usecs_to_jiffies);
+EXPORT_SYMBOL(__usecs_to_jiffies);
 
 /*
  * The TICK_NSEC - 1 rounds up the value to the next resolution.  Note

Re: [PATCH v2 net-next] softirq: reduce latencies

[David Miller]

From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Thu, 03 Jan 2013 23:49:40 -0800

> From: Eric Dumazet <edumazet@google.com>
> 
> In various network workloads, __do_softirq() latencies can be up
> to 20 ms if HZ=1000, and 200 ms if HZ=100.
> 
> This is because we iterate 10 times in the softirq dispatcher,
> and some actions can consume a lot of cycles.
> 
> This patch changes the fallback to ksoftirqd condition to :
> 
> - A time limit of 2 ms.
> - need_resched() being set on current task
> 
> When one of this condition is met, we wakeup ksoftirqd for further
> softirq processing if we still have pending softirqs.
 ...
> Signed-off-by: Eric Dumazet <edumazet@google.com>

Acked-by: David S. Miller <davem@davemloft.net>

Re: [PATCH v2 net-next] softirq: reduce latencies

[Sedat Dilek]

[ QUOTE ]

From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Thu, 03 Jan 2013 23:49:40 -0800

> From: Eric Dumazet <edumazet@google.com>
>
> In various network workloads, __do_softirq() latencies can be up
> to 20 ms if HZ=1000, and 200 ms if HZ=100.
>
> This is because we iterate 10 times in the softirq dispatcher,
> and some actions can consume a lot of cycles.
>
> This patch changes the fallback to ksoftirqd condition to :
>
> - A time limit of 2 ms.
> - need_resched() being set on current task
>
> When one of this condition is met, we wakeup ksoftirqd for further
> softirq processing if we still have pending softirqs.
 ...
> Signed-off-by: Eric Dumazet <edumazet@google.com>

Acked-by: David S. Miller <davem@davemloft.net>

[ /QUOTE ]

Tested-by: Sedat Dilek <sedat.dilek@gmail.com> (against Linux v3.8-rc2)