linux-kernel - Re: [BUG, bisect] hrtimer: severe lag after suspend & resume

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Date:	Fri, 5 Jun 2015 09:57:34 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Jeremiah Mahler <jmmahler@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [BUG, bisect] hrtimer: severe lag after suspend & resume

* John Stultz <john.stultz@...aro.org> wrote:

> So I suspect the problem is the change to clock_was_set_seq in 
> timekeeping_update is done prior to mirroring the time state to the 
> shadow-timekeeper. Thus the next time we do update_wall_time() the updated 
> sequence is overwritten by whats in the shadow copy. The attached patch moving 
> the modification up seems to avoid the issue for me.
> 
> Thomas: Looking at the problematic change, I'm not a big fan of it. Caching 
> timekeeping state here in the hrtimer code has been a source of bugs in the 
> past, and I'm not sure I see how avoiding copying 24bytes is that big of a win. 
> Especially since it adds more state to the timekeeper and hrtimer base that we 
> have to read and mange. Personally I'd prefer a revert to my fix.

So it's not really the copying of the 24 bytes that is the problem with that 
pattern.

It's the constant dirtying of a cacheline that would otherwise be read-mostly, and 
then the reading of it from the percpu hrtimer interrupts.

That can have negative scalability effects similar to a global lock.

( Now I have not checked whether this cacheline truly becomes read-mostly after 
  the original commit - I suspect Thomas did. )

> +	if (action & TK_CLOCK_WAS_SET)
> +		tk->clock_was_set_seq++;
> +
>  	tk_update_ktime_data(tk);

So I'd also add a comment that this update should be done before:

        if (action & TK_MIRROR)
                memcpy(&shadow_timekeeper, &tk_core.timekeeper,
                       sizeof(tk_core.timekeeper));

Also, there appears to be a layering violation here (unless I mis-read the 
tk_real/tk logic): this should copy 'tk', not access tk_core directly, correct?

Right now this does not matter, because 'tk == &tk_core' is always supposed to be 
true, or at least it should point to an identical shadow copy, right?

But nevertheless tk_core should only ever be directly referenced when 'tk' is 
initialized from it.

Also, there's a few more of these apparent layering violations:

  git grep 'tk_core' kernel/time/timekeeping.c | grep -v 'struct timekeeper'

I realize that we are transitioning from former global variables based timekeeping 
to a more parametric model, so this isn't a complaint, just an observation.

Right now it's a code cleanliness detail, but if/when we introduce clocks that are 
updated independently from each other then it will also matter functionally.

Thanks,

	Ingo
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