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Message-ID: <4420FCFD.3060401@yahoo.com.au>
Date: Wed, 22 Mar 2006 18:30:05 +1100
From: Nick Piggin <nickpiggin@yahoo.com.au>
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To: "Chen, Kenneth W" <kenneth.w.chen@intel.com>
CC: "Li, Shaohua" <shaohua.li@intel.com>,
       "'lkml'" <linux-kernel@vger.kernel.org>,
       "'Andrew Morton'" <akpm@osdl.org>
Subject: Re: [PATCH] less tlb flush in unmap_vmas
References: <200603220715.k2M7F1g04936@unix-os.sc.intel.com>
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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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