Re: [tip: sched/core] sched/tracing: Don't re-read p->state when emitting sched_switch event

[Holger Hoffstätte]

On 2022-04-11 09:28, Greg KH wrote:
> On Mon, Apr 11, 2022 at 09:18:19AM +0200, Holger Hoffstätte wrote:
> > On 2022-04-11 01:22, Holger Hoffstätte wrote:
> > > On 2022-04-11 00:06, Qais Yousef wrote:
> > > > On 04/10/22 00:38, Qais Yousef wrote:
> > > > > On 03/08/22 18:51, Qais Yousef wrote:
> > > > > > On 03/08/22 19:10, Greg KH wrote:
> > > > > > > On Tue, Mar 08, 2022 at 06:02:40PM +0000, Qais Yousef wrote:
> > > > > > > > +CC stable
> > > > > > > >
> > > > > > > > On 03/01/22 15:24, tip-bot2 for Valentin Schneider wrote:
> > > > > > > > > The following commit has been merged into the sched/core branch of tip:
> > > > > > > > >
> > > > > > > > > Commit-ID:     fa2c3254d7cfff5f7a916ab928a562d1165f17bb
> > > > > > > > > Gitweb:        https://git.kernel.org/tip/fa2c3254d7cfff5f7a916ab928a562d1165f17bb
> > > > > > > > > Author:        Valentin Schneider <valentin.schneider@xxxxxxx>
> > > > > > > > > AuthorDate:    Thu, 20 Jan 2022 16:25:19
> > > > > > > > > Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > > > > > > > CommitterDate: Tue, 01 Mar 2022 16:18:39 +01:00
> > > > > > > > >
> > > > > > > > > sched/tracing: Don't re-read p->state when emitting sched_switch event
> > > > > > > > >
> > > > > > > > > As of commit
> > > > > > > > >
> > > > > > > > >     c6e7bd7afaeb ("sched/core: Optimize ttwu() spinning on p->on_cpu")
> > > > > > > > >
> > > > > > > > > the following sequence becomes possible:
> > > > > > > > >
> > > > > > > > >                p->__state = TASK_INTERRUPTIBLE;
> > > > > > > > >                __schedule()
> > > > > > > > >              deactivate_task(p);
> > > > > > > > >     ttwu()
> > > > > > > > >       READ !p->on_rq
> > > > > > > > >       p->__state=TASK_WAKING
> > > > > > > > >              trace_sched_switch()
> > > > > > > > >                __trace_sched_switch_state()
> > > > > > > > >                  task_state_index()
> > > > > > > > >                    return 0;
> > > > > > > > >
> > > > > > > > > TASK_WAKING isn't in TASK_REPORT, so the task appears as TASK_RUNNING in
> > > > > > > > > the trace event.
> > > > > > > > >
> > > > > > > > > Prevent this by pushing the value read from __schedule() down the trace
> > > > > > > > > event.
> > > > > > > > >
> > > > > > > > > Reported-by: Abhijeet Dharmapurikar <adharmap@xxxxxxxxxxx>
> > > > > > > > > Signed-off-by: Valentin Schneider <valentin.schneider@xxxxxxx>
> > > > > > > > > Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > > > > > > > > Reviewed-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
> > > > > > > > > Link: https://lore.kernel.org/r/20220120162520.570782-2-valentin.schneider@xxxxxxx
> > > > > > > >
> > > > > > > > Any objection to picking this for stable? I'm interested in this one for some
> > > > > > > > Android users but prefer if it can be taken by stable rather than backport it
> > > > > > > > individually.
> > > > > > > >
> > > > > > > > I think it makes sense to pick the next one in the series too.
> > > > > > >
> > > > > > > What commit does this fix in Linus's tree?
> > > > > >
> > > > > > It should be this one: c6e7bd7afaeb ("sched/core: Optimize ttwu() spinning on p->on_cpu")
> > > > >
> > > > > Should this be okay to be picked up by stable now? I can see AUTOSEL has picked
> > > > > it up for v5.15+, but it impacts v5.10 too.
> > > >
> > > > commit: fa2c3254d7cfff5f7a916ab928a562d1165f17bb
> > > > subject: sched/tracing: Don't re-read p->state when emitting sched_switch event
> > > >
> > > > This patch has an impact on Android 5.10 users who experience tooling breakage.
> > > > Is it possible to include in 5.10 LTS please?
> > > >
> > > > It was already picked up for 5.15+ by AUTOSEL and only 5.10 is missing.
> > >
> > > https://lore.kernel.org/stable/Yk2PQzynOVOzJdPo@xxxxxxxxx/
> > >
> > > However, since then further investigation (still in progress) has shown that this
> > > may have been the fault of the tool in question, so if you can verify that tracing
> > > sched still works for you with this patch in 5.15.x then by all means
> > > let's merge it.
> >
> > So it turns out the lockup is indeed the fault of the tool, which contains multiple
> > kernel-version dependent tracepoint definitions and now fails with this
> > patch.
>
> What tools is this?

sysdig - which uses a helper kernel module which accesses tracepoints, but of course
(as I just found) with copypasta'd TP definitions, which broke with this patch due to
the additional parameter in the function signature. It's been prone to breakage forever
because of a lack of a stable kernel ABI.

Took me a while to find/figure out, but IMHO better safe than sorry. We've had
autoselected scheduler patches before that looked fine but really were not.

> > Greg, please re-enqueue this patch where necessary (5.10, 5.15+)
>
> If I queue it up again, will the tools keep breaking?

Yes, but that's their problem with an out-of-tree module; a few more #ifdefs
are not going to make a big difference.

thanks
Holger