Hi, better late than never..
On Monday 17 September 2012 22:38:20 Rafael J. Wysocki wrote: > On Monday, September 17, 2012, Tejun Heo wrote: > > powernowk8_target() runs off a per-cpu work item and if the > > cpufreq_policy->cpu is different from the current one, it migrates the > > kworker to the target CPU by manipulating current->cpus_allowed. The > > function migrates the kworker back to the original CPU but this is > > still broken. Workqueue concurrency management requires the kworkers > > to stay on the same CPU and powernowk8_target() ends up triggerring > > BUG_ON(rq != this_rq()) in try_to_wake_up_local() if it contends on > > fidvid_mutex and sleeps. > > > > It is unclear why this bug is being reported now. Duncan says it > > appeared to be a regression of 3.6-rc1 and couldn't reproduce it on > > 3.5. Bisection seemed to point to 63d95a91 "workqueue: use @pool > > instead of @gcwq or @cpu where applicable" which is an non-functional > > change. Given that the reproduce case sometimes took upto days to > > trigger, it's easy to be misled while bisecting. Maybe something made > > contention on fidvid_mutex more likely? I don't know. > > > > This patch fixes the bug by punting to another per-cpu work item on > > the target CPU if it isn't the same as the current one. The code > > assumes that cpufreq_policy->cpu is kept online by the caller, which > > Rafael tells me is the case. > > > > Signed-off-by: Tejun Heo <t...@kernel.org> > > Reported-by: Duncan <1i5t5.dun...@cox.net> > > Cc: Rafael J. Wysocki <r...@sisk.pl> > > Cc: Andreas Herrmann <andreas.herrma...@amd.com> > > Cc: sta...@kernel.org > > Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=47301 > > --- > > > > While it's very late in the merge cycle, the fix is limited in scope > > and fairly safe, so it wouldn't be too crazy to merge but then again > > this can go through the next -rc1 and then -stable. Linus, Rafael, > > what do you guys think? > > Well, I don't see much reason to wait with this, although I'd like some > more people to check it. > > Andre, Thomas, can you please have a look at it? The cpufreq changes are not really (functional) changes. I cannot judge the risk of the real change: > > + INIT_WORK_ONSTACK(&tw.work, powernowk8_target_on_cpu); instead of using set_cpus_allowed_ptr. Changing scheduler behavior of powernow-k8 sounds rather intrusive for rc6, but I would fully trust Tejun's advise on this. I wonder whether more drivers are affected similarly, grepping for: set_cpus_allowed_ptr shows quite some hits. My 2 cents..., Thomas -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
