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Date:   Fri, 15 Mar 2019 17:03:47 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Phil Auld <pauld@redhat.com>
Cc:     linux-kernel@vger.kernel.org, Ben Segall <bsegall@google.com>,
        Ingo Molnar <mingo@redhat.com>
Subject: Re: [PATCH] sched/fair: Limit sched_cfs_period_timer loop to avoid
 hard lockup
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 <20190315153042.GF27131@pauld.bos.csb>
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On Fri, Mar 15, 2019 at 11:30:42AM -0400, Phil Auld wrote:

> In my defense here, all the fair.c imbalance pct code also uses 100 :)

Yes, I know, I hate on that too ;-) Just never got around to fixing
that.


> with the below:
> 
> [  117.235804] cfs_period_timer[cpu2]: period too short, scaling up (new cfs_period_us 2492, cfs_quota_us = 143554)
> [  117.346807] cfs_period_timer[cpu2]: period too short, scaling up (new cfs_period_us 2862, cfs_quota_us = 164863)
> [  117.470569] cfs_period_timer[cpu2]: period too short, scaling up (new cfs_period_us 3286, cfs_quota_us = 189335)
> [  117.574883] cfs_period_timer[cpu2]: period too short, scaling up (new cfs_period_us 3774, cfs_quota_us = 217439)
> [  117.652907] cfs_period_timer[cpu2]: period too short, scaling up (new cfs_period_us 4335, cfs_quota_us = 249716)
> [  118.090535] cfs_period_timer[cpu2]: period too short, scaling up (new cfs_period_us 4978, cfs_quota_us = 286783)
> [  122.098009] cfs_period_timer[cpu2]: period too short, scaling up (new cfs_period_us 5717, cfs_quota_us = 329352)
> [  126.255209] cfs_period_timer[cpu2]: period too short, scaling up (new cfs_period_us 6566, cfs_quota_us = 378240)
> [  126.358060] cfs_period_timer[cpu2]: period too short, scaling up (new cfs_period_us 7540, cfs_quota_us = 434385)
> [  126.538358] cfs_period_timer[cpu9]: period too short, scaling up (new cfs_period_us 8660, cfs_quota_us = 498865)
> [  126.614304] cfs_period_timer[cpu9]: period too short, scaling up (new cfs_period_us 9945, cfs_quota_us = 572915)
> [  126.817085] cfs_period_timer[cpu9]: period too short, scaling up (new cfs_period_us 11422, cfs_quota_us = 657957)
> [  127.352038] cfs_period_timer[cpu9]: period too short, scaling up (new cfs_period_us 13117, cfs_quota_us = 755623)
> [  127.598043] cfs_period_timer[cpu9]: period too short, scaling up (new cfs_period_us 15064, cfs_quota_us = 867785)
> 
> 
> Plus on repeats I see an occasional 
> 
> [  152.803384] sched_cfs_period_timer: 9 callbacks suppressed

That should be fine, right? It's a fallback for an edge case and
shouldn't trigger too often anyway.

>> I'll rework the maths in the averaged version and post v2 if that makes sense.
> 
> It may have the extra timer fetch, although maybe I could rework it so that it used the 
> nsstart time the first time and did not need to do it twice in a row. I had originally
> reverted the hrtimer_forward_now() to hrtimer_forward() but put that back. 

Sure; but remember, simpler is often better, esp. for code that
typically 'never' runs.

> Also, fwiw, this was reported earlier by Anton Blanchard in https://lkml.org/lkml/2018/12/3/1047

Bah, yes, I sometimes loose track of things :/