linux-kernel - Re: [PATCH] x86/mm/tlb: ignore f->new_tlb

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Date:   Fri, 8 Jul 2022 08:13:13 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     David Hildenbrand <david@...hat.com>,
        Nadav Amit <nadav.amit@...il.com>,
        linux-kernel@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, Linux MM <linux-mm@...ck.org>,
        Nadav Amit <namit@...are.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH] x86/mm/tlb: ignore f->new_tlb_gen when zero

On 7/8/22 04:40, David Hildenbrand wrote:
>> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
>> index d9314cc8b81f..d81b4084bb8a 100644
>> --- a/arch/x86/mm/tlb.c
>> +++ b/arch/x86/mm/tlb.c
>> @@ -771,14 +771,14 @@ static void flush_tlb_func(void *info)
>>  		return;
>>  	}
>>  
>> -	if (f->new_tlb_gen <= local_tlb_gen) {
>> +	if (unlikely(f->new_tlb_gen != 0 && f->new_tlb_gen <= local_tlb_gen)) {
>>  		/*
>>  		 * The TLB is already up to date in respect to f->new_tlb_gen.
>>  		 * While the core might be still behind mm_tlb_gen, checking
>>  		 * mm_tlb_gen unnecessarily would have negative caching effects
>>  		 * so avoid it.
>>  		 */
>> -		return;
>> +		goto done;
> Does this affect the performance numbers from aa44284960d5 ("x86/mm/tlb:
> Avoid reading mm_tlb_gen when possible")?

It depends on how many batched flushes that workload had.  From the
looks of it, they're all one page:

	madvise(addr + i, pgsize, MADV_DONTNEED);

so there shouldn't be *much* batching in play.  But, it wouldn't hurt to
re-run them in either case.