Re: [RFC PATCH] tpm: msleep() delays - replace with usleep_range() in i2c nuvoton driver

[Mimi Zohar <zohar-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>]

On Thu, 2017-03-02 at 10:33 +0200, Jarkko Sakkinen wrote:
> On Fri, Feb 24, 2017 at 12:29:02PM -0500, Mimi Zohar wrote:
> > On Fri, 2017-02-24 at 19:01 +0200, Jarkko Sakkinen wrote:
> > > On Thu, Feb 23, 2017 at 06:46:18PM -0500, Mimi Zohar wrote:
> > > > Commit 500462a9de65 "timers: Switch to a non-cascading wheel" replaced
> > > > the 'classic' timer wheel, which aimed for near 'exact' expiry of the
> > > > timers.  Their analysis was that the vast majority of timeout timers
> > > > are used as safeguards, not as real timers, and are cancelled or
> > > > rearmed before expiration.  The only exception noted to this were
> > > > networking timers with a small expiry time.
> > > > 
> > > > Not included in the analysis was the TPM polling timer, which resulted
> > > > in a longer normal delay and, every so often, a very long delay.  The
> > > > non-cascading wheel delay is based on CONFIG_HZ.  For a description of
> > > > the different rings and their delays, refer to the comments in
> > > > kernel/time/timer.c.
> > > > 
> > > > Below are the delays given for rings 0 - 2, which explains the longer
> > > > "normal" delays and the very, long delays as seen on systems with
> > > > CONFIG_HZ 250.
> > > > 
> > > > * HZ 1000 steps
> > > >  * Level Offset  Granularity            Range
> > > >  *  0      0         1 ms                0 ms - 63 ms
> > > >  *  1     64         8 ms               64 ms - 511 ms
> > > >  *  2    128        64 ms              512 ms - 4095 ms (512ms - ~4s)
> > > > 
> > > > * HZ  250
> > > >  * Level Offset  Granularity            Range
> > > >  *  0      0         4 ms                0 ms - 255 ms
> > > >  *  1     64        32 ms              256 ms - 2047 ms (256ms - ~2s)
> > > >  *  2    128       256 ms             2048 ms - 16383 ms (~2s - ~16s)
> > > > 
> > > > Below is a comparison of extending the TPM with 1000 measurements,
> > > > using msleep() vs. usleep_delay() when configured for 1000 hz vs. 250
> > > > hz, before and after commit 500462a9de65.
> > > > 
> > > > 		linux-4.7 | msleep()	usleep_range()
> > > > 1000 hz:	0m44.628s | 1m34.497s	29.243s
> > > > 250 hz:		1m28.510s | 4m49.269s	32.386s
> > > > 
> > > > 		linux-4.7 	| min-max (msleep)  min-max (usleep_range)
> > > > 1000 hz:	0:017 - 2:760s	| 0:015 - 3:967s    0:014 - 0:418s
> > > > 250 hz:		0:028 - 1:954s	| 0:040 - 4:096s    0:016 - 0:816s
> > > > 
> > > > This patch replaces the msleep() with usleep_range() calls in the
> > > > i2c nuvoton driver with a consistent max range value.
> > > > 
> > > > Signed-of-by: Mimi Zohar <zohar-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>
> > > > Reviewed-by: Nayna Jain <nayna-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>
> > > 
> > > So why doesn't it go to level 0 with msleep()?  I quickly skimmed
> > > through __mod_timer() and for me it looked like that level 0 would be
> > > calculated (when it is eventually called starting from msleep()).
> > > What did I miss?
> > 
> > I've just added some printk's in kernel/time/timer.c.  It looks like it
> > is level 0.  The delay seems to be caused by schedule() in
> > schedule_timeout().
> > 
> >         setup_timer_on_stack(&timer, process_timeout, (unsigned
> > long)current);
> >         __mod_timer(&timer, expire, false, false);
> >         schedule();  <===
> >         del_singleshot_timer_sync(&timer);
> > 
> >         /* Remove the timer from the object tracker */
> >         destroy_timer_on_stack(&timer);
> > 
> > 
> > printks output:
> > 124.901002] calc_wheel_index: level 0 timer: c000003fab32b150 expires
> > 4294923520 new expires 4294923520 now 4294923518
> > [  124.901003] __mod_timer: exit timer c000003fab32b1a0 now 4294923518
> > 
> > <  call to schedule()  >
> > 
> > [  128.607463] schedule_timeout: before destroy timer: c000003fab32b150
> > expires 4294923520 now 4294924439   <=== notice that the "now" time is
> > way beyond the expires time.
> >    
> > Mimi
> 
> Hey, I totally forgot this patch! Sorry.
> 
> Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>

Thanks, Jarkko!  From looking at the code and adding some printks, the
TPM extends should have been in level 0.    We were left wondering why
msleep() was performing so poorly.   Unfortunately, we haven't gotten
very far.  Initially we thought it might be in "is_idle", but can't even
confirm that.  Sprinkling "printks" isn't very useful as it changes the
timing.  Even adding "1"  like in the change below improved the
performance a lot as shown in the table below.   Thomas, any
suggestions?

Even with this improvement, it doesn't come near to that of using
usleep_range().  I'm not sure what all this means for the rest of the
msleep()s, but at this point, I do appreciate your upstreaming this
patch.  I hard coded the usleep_range to 300.  Any suggestions on what
the max usleep_range() should be?


           4.10.1         |   4.10.1+ with "+ 1" change
---------------------------------------------------------------
real    3m33.510s       1m29.628s
user   0m1.273s         0m1.296s
sys     0m7.518s         0m5.363s


diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index d7f6984ce682..9d3c2ab7011f 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1504,7 +1504,7 @@ static u64 cmp_next_hrtimer_event(u64 basem, u64 expires)
 u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
 {
 	struct timer_base *base = this_cpu_ptr(&timer_bases[BASE_STD]);
-	u64 expires = KTIME_MAX;
+	u64 expire_time, expires = KTIME_MAX;
 	unsigned long nextevt;
 	bool is_max_delta;
 
@@ -1545,7 +1545,8 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
 	}
 	spin_unlock(&base->lock);
 
-	return cmp_next_hrtimer_event(basem, expires);
+	expire_time = cmp_next_hrtimer_event(basem, expires);
+	return (is_max_delta) ? expire_time : expire_time + 1;
 }

Mimi


------------------------------------------------------------------------------
Announcing the Oxford Dictionaries API! The API offers world-renowned
dictionary content that is easy and intuitive to access. Sign up for an
account today to start using our lexical data to power your apps and
projects. Get started today and enter our developer competition.
http://sdm.link/oxford