Re: [PATCH] x86/mm/tlb: avoid reading mm_tlb_gen when possible

[Nadav Amit]

On Mar 28, 2022, at 3:35 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> On Tue, Mar 22, 2022 at 10:07:57PM +0000, Nadav Amit wrote:
>> From: Nadav Amit <namit@xxxxxxxxxx>
>> 
>> On extreme TLB shootdown storms, the mm's tlb_gen cacheline is highly
>> contended and reading it should (arguably) be avoided as much as
>> possible.
>> 
>> Currently, flush_tlb_func() reads the mm's tlb_gen unconditionally,
>> even when it is not necessary (e.g., the mm was already switched).
>> This is wasteful.
>> 
>> Moreover, one of the existing optimizations is to read mm's tlb_gen to
>> see if there are additional in-flight TLB invalidations and flush the
>> entire TLB in such a case. However, if the request's tlb_gen was already
>> flushed, the benefit of checking the mm's tlb_gen is likely to be offset
>> by the overhead of the check itself.
>> 
>> Running will-it-scale with tlb_flush1_threads show a considerable
>> benefit on 56-core Skylake (up to +24%):
>> 
>> threads		Baseline (v5.17+)	+Patch
>> 1		159960			160202
>> 5		310808			308378 (-0.7%)
>> 10		479110			490728
>> 15		526771			562528
>> 20		534495			587316
>> 25		547462			628296
>> 30		579616			666313
>> 35		594134			701814
>> 40		612288			732967
>> 45		617517			749727
>> 50		637476			735497
>> 55		614363			778913 (+24%)
> 
> Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

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