Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408!

[Michal Hocko <mhocko@kernel.org>]

On Wed 20-01-16 15:57:43, Christoph Lameter wrote:
> On Wed, 20 Jan 2016, Michal Hocko wrote:
> 
> > On Wed 20-01-16 09:55:22, Christoph Lameter wrote:
> > [...]
> > > Subject: vmstat: Remove BUG_ON from vmstat_update
> > >
> > > If we detect that there is nothing to do just set the flag and do not check
> > > if it was already set before. Races really do not matter. If the flag is
> > > set by any code then the shepherd will start dealing with the situation
> > > and reenable the vmstat workers when necessary again.
> > >
> > > Concurrent actions could be onlining and offlining of processors or be a
> > > result of concurrency issues when updating the cpumask from multiple
> > > processors.
> >
> > Now that 7e988032 ("vmstat: make vmstat_updater deferrable again and
> > shut down on idle) is merged the VM_BUG_ON is simply bogus because
> > vmstat_update might "race" with quiet_vmstat. The changelog should
> > reflect that. What about the following wording?
> 
> How can it race if preemption is off?

See below.

> > Since 0eb77e988032 ("vmstat: make vmstat_updater deferrable again and
> > shut down on idle") quiet_vmstat might update cpu_stat_off and mark a
> > particular cpu to be handled by vmstat_shepherd. This might trigger
> > a VM_BUG_ON in vmstat_update because the work item might have been
> > sleeping during the idle period and see the cpu_stat_off updated after
> > the wake up. The VM_BUG_ON is therefore misleading and no more
> > appropriate. Moreover it doesn't really suite any protection from real
> > bugs because vmstat_shepherd will simply reschedule the vmstat_work
> > anytime it sees a particular cpu set or vmstat_update would do the same
> > from the worker context directly. Even when the two would race the
> > result wouldn't be incorrect as the counters update is fully idempotent.
> 
> 
> Hmmm... the vmstat_update can be interrupted while running and the cpu put
> into idle mode? If vmstat_update is running then the cpu is not idle but
> running code. If this is really going on then there is other stuff wrong
> with the idling logic.

The vmstat update might be still waiting for its timer, idle mode started
and kick vmstat_update which might cpumask_test_and_set_cpu. Once the
idle terminates and the originally schedule vmstate_update executes it
sees the bit set and BUG_ON.

> > Fixes: 0eb77e988032 ("vmstat: make vmstat_updater deferrable again and
> > shut down on idle")
> > CC: stable # 4.4+
> 
> ?? There has not been an upstream release with this yet.

Ohh, I thought it made it into 4.4 but you are right it is post 4.4 so
no CC: stable required.

-- 
Michal Hocko
SUSE Labs