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

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Date:   Mon, 13 May 2019 17:41:38 +0000
From:   Nadav Amit <namit@...are.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Yang Shi <yang.shi@...ux.alibaba.com>,
        "jstancek@...hat.com" <jstancek@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Nick Piggin <npiggin@...il.com>,
        Minchan Kim <minchan@...nel.org>, Mel Gorman <mgorman@...e.de>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH] mm: mmu_gather: remove __tlb_reset_range() for force
 flush

> On May 13, 2019, at 4:27 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Mon, May 13, 2019 at 09:21:01AM +0000, Nadav Amit wrote:
>>> On May 13, 2019, at 2:12 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
>>>> The other thing I was thinking of is trying to detect overlap through
>>>> the page-tables themselves, but we have a distinct lack of storage
>>>> there.
>>> 
>>> We might just use some state in the pmd, there's still 2 _pt_pad_[12] in
>>> struct page to 'use'. So we could come up with some tlb generation
>>> scheme that would detect conflict.
>> 
>> It is rather easy to come up with a scheme (and I did similar things) if you
>> flush the table while you hold the page-tables lock. But if you batch across
>> page-tables it becomes harder.
> 
> Yeah; finding that out now. I keep finding holes :/

You can use Uhlig’s dissertation for inspiration (Section 4.4).

[1] https://www.researchgate.net/publication/36450482_Scalability_of_microkernel-based_systems/download

> 
>> Thinking about it while typing, perhaps it is simpler than I think - if you
>> need to flush range that runs across more than a single table, you are very
>> likely to flush a range of more than 33 entries, so anyhow you are likely to
>> do a full TLB flush.
> 
> We can't rely on the 33, that x86 specific. Other architectures can have
> another (or no) limit on that.

I wonder whether there are architectures that do no invalidate the TLB entry
by entry, experiencing these kind of overheads.

>> So perhaps just avoiding the batching if only entries from a single table
>> are flushed would be enough.
> 
> That's near to what Will suggested initially, just flush the entire
> thing when there's a conflict.

One question is how you define a conflict. IIUC, Will suggests same mm marks
a conflict. In addition, I suggest that if you only remove a single entry
(or few ones), you would just not batch and do the flushing while holding
the page-table lock.