On Mon, Sep 17, 2012 at 01:17:21PM -0700, 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 <tj@xxxxxxxxxx> > Reported-by: Duncan <1i5t5.duncan@xxxxxxx> > Cc: Rafael J. Wysocki <rjw@xxxxxxx> > Cc: Andreas Herrmann <andreas.herrmann3@xxxxxxx> > Cc: stable@xxxxxxxxxx > 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? Wouldn't it be much simpler to carve out the piece after set_cpus_allowed_ptr(), put it in a sub-function called __powernowk8_target() and call it with smp_call_function_single instead of defining another work item? Would the workqueue code handle that or are there any other issues? > drivers/cpufreq/powernow-k8.c | 89 +++++++++++++++++++++++++----------------- > 1 file changed, 53 insertions(+), 36 deletions(-) If it can, the diffstat should look much slimmer. Thanks. -- Regards/Gruss, Boris. -- To unsubscribe from this list: send the line "unsubscribe cpufreq" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
