Re: BUG: scheduling while atomic: cron/668/0x10c9a0c0

[Mel Gorman]

On Thu, Jun 02, 2016 at 02:04:42PM +0200, Vlastimil Babka wrote:
> On 06/02/2016 12:39 PM, Mel Gorman wrote:
> >On Wed, Jun 01, 2016 at 12:01:24PM +0200, Vlastimil Babka wrote:
> >>>Why?
> >>>
> >>>The comment is fine but I do not see why the recalculation would occur.
> >>>
> >>>In the original code, the preferred_zoneref for statistics is calculated
> >>>based on either the supplied nodemask or cpuset_current_mems_allowed during
> >>>the initial attempt. It then relies on the cpuset checks in the slowpath
> >>>to encorce mems_allowed but the preferred zone doesn't change.
> >>>
> >>>With your proposed change, it's possible that the
> >>>preferred_zoneref recalculation points to a zoneref disallowed by
> >>>cpuset_current_mems_sllowed. While it'll be skipped during allocation,
> >>>the statistics will still be against a zone that is potentially outside
> >>>what is allowed.
> >>
> >>Hmm that's true and I was ready to agree. But then I noticed  that
> >>gfp_to_alloc_flags() can mask out ALLOC_CPUSET for GFP_ATOMIC. So it's
> >>like a lighter version of the ALLOC_NO_WATERMARKS situation. In that
> >>case it's wrong if we leave ac->preferred_zoneref at a position that has
> >>skipped some zones due to mempolicies?
> >>
> >
> >So both options are wrong then. How about this?
> 
> I wonder if the original patch we're fixing was worth all this trouble (and
> more
> for my compaction priority series :), but yeah this should work.
> 

I considered that option when the bug report first came in. It was a 2%
hit to the page allocator microbenchmark to revert it. More than I expected
but enough to care. If this causes another problem, I'll revert it as
there will be other options later.

> >---8<---
> >mm, page_alloc: Recalculate the preferred zoneref if the context can ignore memory policies
> >
> >The optimistic fast path may use cpuset_current_mems_allowed instead of
> >of a NULL nodemask supplied by the caller for cpuset allocations. The
> >preferred zone is calculated on this basis for statistic purposes and
> >as a starting point in the zonelist iterator.
> >
> >However, if the context can ignore memory policies due to being atomic or
> >being able to ignore watermarks then the starting point in the zonelist
> >iterator is no longer correct. This patch resets the zonelist iterator in
> >the allocator slowpath if the context can ignore memory policies. This will
> >alter the zone used for statistics but only after it is known that it makes
> >sense for that context. Resetting it before entering the slowpath would
> >potentially allow an ALLOC_CPUSET allocation to be accounted for against
> >the wrong zone. Note that while nodemask is not explicitly set to the
> >original nodemask, it would only have been overwritten if cpuset_enabled()
> >and it was reset before the slowpath was entered.
> >
> >Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> 
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> 

Thanks.

-- 
Mel Gorman
SUSE Labs