From: Nadav Amit <namit@vmware.com>
To: Will Deacon <will.deacon@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
Yang Shi <yang.shi@linux.alibaba.com>,
"jstancek@redhat.com" <jstancek@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>,
"stable@vger.kernel.org" <stable@vger.kernel.org>,
Linux-MM <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>,
"Aneesh Kumar K . V" <aneesh.kumar@linux.vnet.ibm.com>,
Nick Piggin <npiggin@gmail.com>, Minchan Kim <minchan@kernel.org>,
Mel Gorman <mgorman@suse.de>
Subject: Re: [PATCH] mm: mmu_gather: remove __tlb_reset_range() for force flush
Date: Mon, 13 May 2019 17:06:03 +0000 [thread overview]
Message-ID: <43638259-8EDB-4B8D-A93D-A2E86D8B2489@vmware.com> (raw)
In-Reply-To: <20190513163752.GA10754@fuggles.cambridge.arm.com>
> On May 13, 2019, at 9:37 AM, Will Deacon <will.deacon@arm.com> wrote:
>
> On Mon, May 13, 2019 at 09:11:38AM +0000, Nadav Amit wrote:
>>> On May 13, 2019, at 1:36 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>>>
>>> On Thu, May 09, 2019 at 09:21:35PM +0000, Nadav Amit wrote:
>>>
>>>>>>> And we can fix that by having tlb_finish_mmu() sync up. Never let a
>>>>>>> concurrent tlb_finish_mmu() complete until all concurrenct mmu_gathers
>>>>>>> have completed.
>>>>>>>
>>>>>>> This should not be too hard to make happen.
>>>>>>
>>>>>> This synchronization sounds much more expensive than what I proposed. But I
>>>>>> agree that cache-lines that move from one CPU to another might become an
>>>>>> issue. But I think that the scheme I suggested would minimize this overhead.
>>>>>
>>>>> Well, it would have a lot more unconditional atomic ops. My scheme only
>>>>> waits when there is actual concurrency.
>>>>
>>>> Well, something has to give. I didn’t think that if the same core does the
>>>> atomic op it would be too expensive.
>>>
>>> They're still at least 20 cycles a pop, uncontended.
>>>
>>>>> I _think_ something like the below ought to work, but its not even been
>>>>> near a compiler. The only problem is the unconditional wakeup; we can
>>>>> play games to avoid that if we want to continue with this.
>>>>>
>>>>> Ideally we'd only do this when there's been actual overlap, but I've not
>>>>> found a sensible way to detect that.
>>>>>
>>>>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>>>>> index 4ef4bbe78a1d..b70e35792d29 100644
>>>>> --- a/include/linux/mm_types.h
>>>>> +++ b/include/linux/mm_types.h
>>>>> @@ -590,7 +590,12 @@ static inline void dec_tlb_flush_pending(struct mm_struct *mm)
>>>>> *
>>>>> * Therefore we must rely on tlb_flush_*() to guarantee order.
>>>>> */
>>>>> - atomic_dec(&mm->tlb_flush_pending);
>>>>> + if (atomic_dec_and_test(&mm->tlb_flush_pending)) {
>>>>> + wake_up_var(&mm->tlb_flush_pending);
>>>>> + } else {
>>>>> + wait_event_var(&mm->tlb_flush_pending,
>>>>> + !atomic_read_acquire(&mm->tlb_flush_pending));
>>>>> + }
>>>>> }
>>>>
>>>> It still seems very expensive to me, at least for certain workloads (e.g.,
>>>> Apache with multithreaded MPM).
>>>
>>> Is that Apache-MPM workload triggering this lots? Having a known
>>> benchmark for this stuff is good for when someone has time to play with
>>> things.
>>
>> Setting Apache2 with mpm_worker causes every request to go through
>> mmap-writev-munmap flow on every thread. I didn’t run this workload after
>> the patches that downgrade the mmap_sem to read before the page-table
>> zapping were introduced. I presume these patches would allow the page-table
>> zapping to be done concurrently, and therefore would hit this flow.
>
> Hmm, I don't think so: munmap() still has to take the semaphore for write
> initially, so it will be serialised against other munmap() threads even
> after they've downgraded afaict.
>
> The initial bug report was about concurrent madvise() vs munmap().
I guess you are right (and I’m wrong).
Short search suggests that ebizzy might be affected (a thread by Mel
Gorman): https://lkml.org/lkml/2015/2/2/493
next prev parent reply other threads:[~2019-05-13 17:06 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-07 21:34 [PATCH] mm: mmu_gather: remove __tlb_reset_range() for force flush Yang Shi
2019-05-09 8:37 ` Will Deacon
2019-05-09 10:38 ` Peter Zijlstra
2019-05-09 10:54 ` Peter Zijlstra
2019-05-09 18:35 ` Yang Shi
2019-05-09 18:40 ` Peter Zijlstra
2019-05-09 12:44 ` Jan Stancek
2019-05-09 17:36 ` Nadav Amit
2019-05-09 18:24 ` Peter Zijlstra
2019-05-09 19:10 ` Yang Shi
2019-05-09 21:06 ` Jan Stancek
2019-05-09 21:48 ` Yang Shi
2019-05-09 22:12 ` Jan Stancek
[not found] ` <04668E51-FD87-4D53-A066-5A35ABC3A0D6@vmware.com>
[not found] ` <20190509191120.GD2623@hirez.programming.kicks-ass.net>
2019-05-09 21:21 ` Nadav Amit
2019-05-13 8:36 ` Peter Zijlstra
2019-05-13 9:11 ` Nadav Amit
2019-05-13 11:30 ` Peter Zijlstra
2019-05-13 16:37 ` Will Deacon
2019-05-13 17:06 ` Nadav Amit [this message]
2019-05-14 8:58 ` Mel Gorman
2019-05-13 9:12 ` Peter Zijlstra
2019-05-13 9:21 ` Nadav Amit
2019-05-13 11:27 ` Peter Zijlstra
2019-05-13 17:41 ` Nadav Amit
2019-05-09 18:22 ` Yang Shi
2019-05-09 19:56 ` Peter Zijlstra
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