linux-kernel - Re: [External] Re: Fwd: WARNING: CPU: 13 PID: 3837105 at kernel/sched/sched.h:1561 __cfsb_csd

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Message-ID: <171e6a9435a33885a73b48762f86954e447c26c2.camel@linux.intel.com>
Date:   Thu, 07 Sep 2023 14:01:10 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Hao Jia <jiahao.os@...edance.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Benjamin Segall <bsegall@...gle.com>,
        Bagas Sanjaya <bagasdotme@...il.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Igor Raits <igor.raits@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Regressions <regressions@...ts.linux.dev>,
        Linux Stable <stable@...r.kernel.org>
Subject: Re: [External] Re: Fwd: WARNING: CPU: 13 PID: 3837105 at
 kernel/sched/sched.h:1561 __cfsb_csd_unthrottle+0x149/0x160

On Thu, 2023-09-07 at 16:59 +0800, Hao Jia wrote:
> 
> On 2023/9/5 Peter Zijlstra wrote:
> > On Thu, Aug 31, 2023 at 04:48:29PM +0800, Hao Jia wrote:
> > 
> > > If I understand correctly, rq->clock_update_flags may be set to
> > > RQCF_ACT_SKIP after __schedule() holds the rq lock, and sometimes the rq
> > > lock may be released briefly in __schedule(), such as newidle_balance(). At
> > > this time Other CPUs hold this rq lock, and then calling
> > > rq_clock_start_loop_update() may trigger this warning.
> > > 
> > > This warning check might be wrong. We need to add assert_clock_updated() to
> > > check that the rq clock has been updated before calling
> > > rq_clock_start_loop_update().
> > > 
> > > Maybe some things can be like this?
> > 
> > Urgh, aside from it being white space mangled, I think this is entirely
> > going in the wrong direction.
> > 
> > Leaking ACT_SKIP is dodgy as heck.. it's entirely too late to think
> > clearly though, I'll have to try again tomorrow.

I am trying to understand why this is an ACT_SKIP leak.
Before call to __cfsb_csd_unthrottle(), is it possible someone
else lock the runqueue, set ACT_SKIP and release rq_lock?
And then that someone never update the rq_clock? 

> 
> Hi Peter,
> 
> Do you think this fix method is correct? Or should we go back to the 
> beginning and move update_rq_clock() from unthrottle_cfs_rq()?
> 
If anyone who locked the runqueue set ACT_SKIP also will update rq_clock,
I think your change is okay.  Otherwise rq_clock could be missing update.

Thanks.

Tim