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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