Re: [PATCH] less tlb flush in unmap_vmas

[Nick Piggin <nickpiggin@yahoo.com.au>]

Chen, Kenneth W wrote:
> Nick Piggin wrote on Tuesday, March 21, 2006 8:53 PM
>
> > Shaohua Li wrote:
> >
> > > In unmaping region, if current task doesn't need reschedule, don't do a
> > > tlb_finish_mmu. This can reduce some tlb flushes.
> > >
> > > In the lmbench tests, this patch gives 2.1% improvement on exec proc
> > > item and 4.2% on sh proc item.
> >
> > The problem with this is that by the time we _do_ determine that a
> > reschedule is needed, we might have built up a huge amount of work
> > to do (which can probably be as much if not more exensive per-page
> > as the unmapping), so scheduling latency can still be unacceptable
> > so I'm afraid I don't think we can include this patch.
>
>
> Interesting. In the old day, since mm->page_table_lock is held for the
> entire unmap_vmas function, it was beneficial to introduce periodic
> reschedule point and to drop the spin lock under pressure. Now that the
> page table lock is fine-grained and is pushed into zap_pte_range(), I
> would think scheduling latency would improve from lock contention
> avoidance point of view.  It is not the case?

Well mmu_gather uses a per-cpu data structure and is non preemptible,
which I guess is one of the main reasons why we have this preemption
here.

You're right that another good reason would be ptl lock contention,
however I don't think that alleviating that problem alone would allow
longer mmu_gather scheduling latencies, because the longest latency
is still the mmu_gather <--> mmu_finish span.

--
SUSE Labs, Novell Inc.
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