On Tue 05-04-16 17:55:39, Hugh Dickins wrote:
> On Tue, 5 Apr 2016, Andrew Morton wrote:
> > On Tue, 5 Apr 2016 13:25:31 +0200 Michal Hocko <mho...@kernel.org> wrote:
> > > - if (is_thp_gfp_mask(gfp_mask)) {
> > > - /*
> > > - * If compaction is deferred for high-order allocations, it is
> > > - * because sync compaction recently failed. If this is the case
> > > - * and the caller requested a THP allocation, we do not want
> > > - * to heavily disrupt the system, so we fail the allocation
> > > - * instead of entering direct reclaim.
> > > - */
> > > - if (compact_result == COMPACT_DEFERRED)
> > > - goto nopage;
> > > -
> > > - /*
> > > - * Compaction is contended so rather back off than cause
> > > - * excessive stalls.
> > > - */
> > > - if(compact_result == COMPACT_CONTENDED)
> > > - goto nopage;
> > > - }
> > > + /*
> > > + * Checks for THP-specific high-order allocations and back off
> > > + * if the the compaction backed off
> > > + */
> > > + if (is_thp_gfp_mask(gfp_mask) && compaction_withdrawn(compact_result))
> > > + goto nopage;
> >
> > This change smashed into Hugh's "huge tmpfs: shmem_huge_gfpmask and
> > shmem_recovery_gfpmask".
> >
> > I ended up doing this:
> >
> > /* Checks for THP-specific high-order allocations */
> > if (!is_thp_allocation(gfp_mask, order))
> > migration_mode = MIGRATE_SYNC_LIGHT;
> >
> > /*
> > * Checks for THP-specific high-order allocations and back off
> > * if the the compaction backed off
> > */
> > if (is_thp_allocation(gfp_mask) && compaction_withdrawn(compact_result))
> > goto nopage;
>
> You'll already have found that is_thp_allocation() needs the order too.
> But then you had to drop a hunk out of his 10/11 also to fit with mine.
>
> What you've done may be just right, but I haven't had time to digest
> Michal's changes yet (and not yet seen what happens to the PF_KTHREAD
> distinction), so I think it will probably end up better if you take
> his exactly as he tested and posted them, and drop my 30/31 and 31/31
> for now
I have only briefly checked your patch30 but I guess the above is
not really necessary. If the request is __GFP_REPEAT (I haven't checked
whether that is the case for shmem) then we promote to MIGRATE_SYNC_LIGHT
as soon as we cannot move on with ASYNC. For !__GFP_REPEAT I did
+ if (is_thp_gfp_mask(gfp_mask) && !(current->flags & PF_KTHREAD))
+ migration_mode = MIGRATE_ASYNC;
+ else
+ migration_mode = MIGRATE_SYNC_LIGHT;
page = __alloc_pages_direct_compact(gfp_mask, order, alloc_flags,
ac, migration_mode,
&compact_result);
so you will end up doing SYNC_LIGHT for !is_thp_allocation as well
> - I can resubmit them (or maybe drop 30 altogether) after I've
> pondered and tested a little on top of Michal's.
I guess this would be safer. If it turns out that we need some special
handling I would prefer if that could be done in should_compact_retry.
> Huge tmpfs got along fine for many months without 30/31 and 31/31: 30
> is just for experimentation, and 31 to reduce the compaction stalls we
> saw under some loads. Maybe I'll find that Michal's rework has changed
> the balance there anyway, and something else or nothing at all needed.
>
> (The gfp_mask stuff was very confusing, and it's painful for me, how
> ~__GFP_KSWAPD_RECLAIM gets used as a secret password to say "THP" and
> how to angle compaction - or maybe it's all more straightforward now.)
>
> Many thanks for giving us both this quick exposure!
Thanks!
--
Michal Hocko
SUSE Labs
