Re: [v2 PATCH] mm: mmu_gather: remove __tlb_reset_range() for force flush

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On 5/13/19 7:01 PM, Nadav Amit wrote:


On May 13, 2019 4:01 PM, Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> wrote:


On 5/13/19 9:38 AM, Will Deacon wrote:
> On Fri, May 10, 2019 at 07:26:54AM +0800, Yang Shi wrote:
>> diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
>> index 99740e1..469492d 100644
>> --- a/mm/mmu_gather.c
>> +++ b/mm/mmu_gather.c
>> @@ -245,14 +245,39 @@ void tlb_finish_mmu(struct mmu_gather *tlb,
>>   {
>>       /*
>>        * If there are parallel threads are doing PTE changes on same range
>> -     * under non-exclusive lock(e.g., mmap_sem read-side) but defer TLB
>> -     * flush by batching, a thread has stable TLB entry can fail to flush
>> -     * the TLB by observing pte_none|!pte_dirty, for example so flush TLB
>> -     * forcefully if we detect parallel PTE batching threads.
>> +     * under non-exclusive lock (e.g., mmap_sem read-side) but defer TLB
>> +     * flush by batching, one thread may end up seeing inconsistent PTEs
>> +     * and result in having stale TLB entries.  So flush TLB forcefully
>> +     * if we detect parallel PTE batching threads.
>> +     *
>> +     * However, some syscalls, e.g. munmap(), may free page tables, this
>> +     * needs force flush everything in the given range. Otherwise this
>> +     * may result in having stale TLB entries for some architectures,
>> +     * e.g. aarch64, that could specify flush what level TLB.
>>        */
>> -    if (mm_tlb_flush_nested(tlb->mm)) {
>> -            __tlb_reset_range(tlb);
>> -            __tlb_adjust_range(tlb, start, end - start);
>> +    if (mm_tlb_flush_nested(tlb->mm) && !tlb->fullmm) {
>> +            /*
>> +             * Since we can't tell what we actually should have
>> +             * flushed, flush everything in the given range.
>> +             */
>> +            tlb->freed_tables = 1;
>> +            tlb->cleared_ptes = 1;
>> +            tlb->cleared_pmds = 1;
>> +            tlb->cleared_puds = 1;
>> +            tlb->cleared_p4ds = 1;
>> +
>> +            /*
>> +             * Some architectures, e.g. ARM, that have range invalidation
>> +             * and care about VM_EXEC for I-Cache invalidation, need force
>> +             * vma_exec set.
>> +             */
>> +            tlb->vma_exec = 1;
>> +
>> +            /* Force vma_huge clear to guarantee safer flush */
>> +            tlb->vma_huge = 0;
>> +
>> +            tlb->start = start;
>> +            tlb->end = end;
>>       }
> Whilst I think this is correct, it would be interesting to see whether
> or not it's actually faster than just nuking the whole mm, as I mentioned
> before.
>
> At least in terms of getting a short-term fix, I'd prefer the diff below
> if it's not measurably worse.

I did a quick test with ebizzy (96 threads with 5 iterations) on my x86
VM, it shows slightly slowdown on records/s but much more sys time spent
with fullmm flush, the below is the data.

                                     nofullmm                 fullmm
ops (records/s)              225606                  225119
sys (s)                            0.69                        1.14

It looks the slight reduction of records/s is caused by the increase of
sys time.

>
> Will
>
> --->8
>
> diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
> index 99740e1dd273..cc251422d307 100644
> --- a/mm/mmu_gather.c
> +++ b/mm/mmu_gather.c
> @@ -251,8 +251,9 @@ void tlb_finish_mmu(struct mmu_gather *tlb,
>         * forcefully if we detect parallel PTE batching threads.
>         */
>        if (mm_tlb_flush_nested(tlb->mm)) {
> +             tlb->fullmm = 1;
>                __tlb_reset_range(tlb);
> -             __tlb_adjust_range(tlb, start, end - start);
> +             tlb->freed_tables = 1;
>        }
>  
>        tlb_flush_mmu(tlb);


I think that this should have set need_flush_all and not fullmm.

Thanks for the suggestion. I did a quick test with ebizzy too. It looks this is almost same with the v2 patch and slightly better than what Will suggested.

                                     nofullmm                 fullmm                need_flush_all
ops (records/s)              225606                  225119                   225647
sys (s)                            0.69                        1.14                          0.47

If no objection from other folks, I would respin the patch based off Nadav's suggestion.




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