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

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

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


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