Comment 3 for bug 1667567

------- Comment From <email address hidden> 2017-02-24 15:35 EDT-------
From <email address hidden> Fri Feb 24 11:30:55 2017
From: Mimi Zohar <email address hidden>
To: Jarkko Sakkinen <email address hidden>
Date: Fri, 24 Feb 2017 12:29:02 -0500
Message-Id: <email address hidden>
Cc: Morav <email address hidden>, <email address hidden>,
<email address hidden>, linux-ima-devel
<email address hidden>, linux-security-module
<email address hidden>, tpmdd-devel
<email address hidden>, linux-fsdevel
<email address hidden>, Gleixner <email address hidden>
Subject: Re: [tpmdd-devel] [RFC PATCH] tpm: msleep() delays - replace with
usleep_range() in i2c nuvoton driver

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 <email address hidden>
> > Reviewed-by: Nayna Jain <email address hidden>
>
> 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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