linux-kernel - Re: [v2 PATCH] mm: mmu_gather: remove __tlb_reset

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Date:   Mon, 20 May 2019 10:59:07 +0800
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Jan Stancek <jstancek@...hat.com>,
        Will Deacon <will.deacon@....com>
Cc:     peterz@...radead.org, namit@...are.com, minchan@...nel.org,
        mgorman@...e.de, stable@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH] mm: mmu_gather: remove __tlb_reset_range() for force
 flush



On 5/16/19 11:29 PM, Jan Stancek wrote:
>
> ----- Original Message -----
>> On Mon, May 13, 2019 at 04:01:09PM -0700, Yang Shi 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.
>> That's not what I expected, and I'm unable to explain why moving to fullmm
>> would /increase/ the system time. I would've thought the time spent doing
>> the invalidation would decrease, with the downside that the TLB is cold
>> when returning back to userspace.
>>
> I tried ebizzy with various parameters (malloc vs mmap, ran it for hour),
> but performance was very similar for both patches.
>
> So, I was looking for workload that would demonstrate the largest difference.
> Inspired by python xml-rpc, which can handle each request in new thread,
> I tried following [1]:
>
> 16 threads, each looping 100k times over:
>    mmap(16M)
>    touch 1 page
>    madvise(DONTNEED)
>    munmap(16M)
>
> This yields quite significant difference for 2 patches when running on
> my 46 CPU arm host. I checked it twice - applied patch, recompiled, rebooted,
> but numbers stayed +- couple seconds the same.

Thanks for the testing. I'm a little bit surprised by the significant 
difference.

I did the same test on my x86 VM (24 cores), they yield almost same number.

Given the significant improvement on arm64 with fullmm version, I'm 
going to respin the patch.

>
> Does it somewhat match your expectation?
>
> v2 patch
> ---------
> real    2m33.460s
> user    0m3.359s
> sys     15m32.307s
>
> real    2m33.895s
> user    0m2.749s
> sys     16m34.500s
>
> real    2m35.666s
> user    0m3.528s
> sys     15m23.377s
>
> real    2m32.898s
> user    0m2.789s
> sys     16m18.801s
>
> real    2m33.087s
> user    0m3.565s
> sys     16m23.815s
>
>
> fullmm version
> ---------------
> real    0m46.811s
> user    0m1.596s
> sys     1m47.500s
>
> real    0m47.322s
> user    0m1.803s
> sys     1m48.449s
>
> real    0m46.668s
> user    0m1.508s
> sys     1m47.352s
>
> real    0m46.742s
> user    0m2.007s
> sys     1m47.217s
>
> real    0m46.948s
> user    0m1.785s
> sys     1m47.906s
>
> [1] https://github.com/jstancek/reproducers/blob/master/kernel/page_fault_stall/mmap8.c