Re: [PATCH] lib/spinlock_debug: Tweak the loop time to fit different _delay()

Re: [PATCH] lib/spinlock_debug: Tweak the loop time to fit different _delay()

[Peter Zijlstra]

On Wed, Apr 30, 2014 at 06:40:38PM -0400, Wang, Xiaoming wrote:
> loops_per_jiffy*Hz  is not always 1 second exactly
> it depends on the realization of _delay() .
> delay_tsc is used as _delay() in arch/x86/lib/delay.c
> It makes loop loops_per_jiffy larger than exception
> and causes one thread can not obtain the spin lock for
> a long time which may trigger HARD LOCKUP in this case.
> So we use cpu_clock() which is more accurate.
> 
> Signed-off-by: Chuansheng Liu <chuansheng.liu@intel.com>
> Signed-off-by: xiaoming wang <xiaoming.wang@intel.com>
> ---
>  kernel/locking/spinlock_debug.c |    9 ++++++---
>  1 files changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/locking/spinlock_debug.c b/kernel/locking/spinlock_debug.c
> index 0374a59..471d26c 100644
> --- a/kernel/locking/spinlock_debug.c
> +++ b/kernel/locking/spinlock_debug.c
> @@ -105,10 +105,13 @@ static inline void debug_spin_unlock(raw_spinlock_t *lock)
>  
>  static void __spin_lock_debug(raw_spinlock_t *lock)
>  {
> -	u64 i;
> -	u64 loops = loops_per_jiffy * HZ;
> +	u64 t;
> +	u64 one_second = 1000000000;
> +	u32 this_cpu = raw_smp_processor_id();
> +
> +	t = cpu_clock(this_cpu);
>  
> -	for (i = 0; i < loops; i++) {
> +	while (cpu_clock(this_cpu) - t < one_second) {
>  		if (arch_spin_trylock(&lock->raw_lock))
>  			return;
>  		__delay(1);

Yep, and now you've broken support for archs that fall back to jiffies
for cpu_clock :-), jiffies need not progress if you've got IRQs
disabled.

Re: [PATCH] lib/spinlock_debug: Tweak the loop time to fit different _delay()

[Peter Zijlstra]

On Wed, Apr 30, 2014 at 06:40:38PM -0400, Wang, Xiaoming wrote:
> loops_per_jiffy*Hz  is not always 1 second exactly
> it depends on the realization of _delay() .
> delay_tsc is used as _delay() in arch/x86/lib/delay.c

This just states delay() is broken. The primary response should be to
try and fix that, no?

RE: [PATCH] lib/spinlock_debug: Tweak the loop time to fit different _delay()

[Liu, Chuansheng]

Hello Peter,

> -----Original Message-----
> From: Peter Zijlstra [mailto:peterz@infradead.org]
> Sent: Wednesday, April 30, 2014 4:55 PM
> To: Wang, Xiaoming
> Cc: mingo@redhat.com; linux-kernel@vger.kernel.org; Liu, Chuansheng
> Subject: Re: [PATCH] lib/spinlock_debug: Tweak the loop time to fit different
> _delay()
> 
> On Wed, Apr 30, 2014 at 06:40:38PM -0400, Wang, Xiaoming wrote:
> > loops_per_jiffy*Hz  is not always 1 second exactly
> > it depends on the realization of _delay() .
> > delay_tsc is used as _delay() in arch/x86/lib/delay.c
> 
> This just states delay() is broken. The primary response should be to
> try and fix that, no?


delay(1s_count) is accurate, but delay(1) is not accurate indeed, since executing
some instruction, then the 1 cycle delay maybe be used already.

Re: [PATCH] lib/spinlock_debug: Tweak the loop time to fit different _delay()

[Peter Zijlstra]

On Wed, Apr 30, 2014 at 09:30:55AM +0000, Liu, Chuansheng wrote:
> > From: Peter Zijlstra [mailto:peterz@infradead.org]
> > Sent: Wednesday, April 30, 2014 4:55 PM
> > To: Wang, Xiaoming
> > Cc: mingo@redhat.com; linux-kernel@vger.kernel.org; Liu, Chuansheng
> > Subject: Re: [PATCH] lib/spinlock_debug: Tweak the loop time to fit different
> > _delay()
> > 
> > On Wed, Apr 30, 2014 at 06:40:38PM -0400, Wang, Xiaoming wrote:
> > > loops_per_jiffy*Hz  is not always 1 second exactly
> > > it depends on the realization of _delay() .
> > > delay_tsc is used as _delay() in arch/x86/lib/delay.c
> > 
> > This just states delay() is broken. The primary response should be to
> > try and fix that, no?
> 
> 
> delay(1s_count) is accurate, but delay(1) is not accurate indeed, since executing
> some instruction, then the 1 cycle delay maybe be used already.

OK, so there's (finally) a problem statement, so is there anything sane
we can do about that?

But yes, a trylock is a cmpxchg, and a cmpxchg on a contended cacheline
can be _much_ longer than one loop.

Now the real problem is coming up with something that'll work for all
architectures.

[PATCH] lib/spinlock_debug: Tweak the loop time to fit different _delay()

[Wang, Xiaoming]

loops_per_jiffy*Hz  is not always 1 second exactly
it depends on the realization of _delay() .
delay_tsc is used as _delay() in arch/x86/lib/delay.c
It makes loop loops_per_jiffy larger than exception
and causes one thread can not obtain the spin lock for
a long time which may trigger HARD LOCKUP in this case.
So we use cpu_clock() which is more accurate.

Signed-off-by: Chuansheng Liu <chuansheng.liu@intel.com>
Signed-off-by: xiaoming wang <xiaoming.wang@intel.com>
---
 kernel/locking/spinlock_debug.c |    9 ++++++---
 1 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/spinlock_debug.c b/kernel/locking/spinlock_debug.c
index 0374a59..471d26c 100644
--- a/kernel/locking/spinlock_debug.c
+++ b/kernel/locking/spinlock_debug.c
@@ -105,10 +105,13 @@ static inline void debug_spin_unlock(raw_spinlock_t *lock)
 
 static void __spin_lock_debug(raw_spinlock_t *lock)
 {
-	u64 i;
-	u64 loops = loops_per_jiffy * HZ;
+	u64 t;
+	u64 one_second = 1000000000;
+	u32 this_cpu = raw_smp_processor_id();
+
+	t = cpu_clock(this_cpu);
 
-	for (i = 0; i < loops; i++) {
+	while (cpu_clock(this_cpu) - t < one_second) {
 		if (arch_spin_trylock(&lock->raw_lock))
 			return;
 		__delay(1);
-- 
1.7.1