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From: Nadav Amit <namit@vmware.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: 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>, Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH] mm: mmu_gather: remove __tlb_reset_range() for force flush
Date: Mon, 13 May 2019 09:21:01 +0000	[thread overview]
Message-ID: <847D4C2F-BD26-4BE0-A5BA-3C690D11BF77@vmware.com> (raw)
In-Reply-To: <20190513091205.GO2650@hirez.programming.kicks-ass.net>

> On May 13, 2019, at 2:12 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> 
> On Mon, May 13, 2019 at 10:36:06AM +0200, Peter Zijlstra wrote:
>> On Thu, May 09, 2019 at 09:21:35PM +0000, Nadav Amit wrote:
>>> It may be possible to avoid false-positive nesting indications (when the
>>> flushes do not overlap) by creating a new struct mmu_gather_pending, with
>>> something like:
>>> 
>>>  struct mmu_gather_pending {
>>> 	u64 start;
>>> 	u64 end;
>>> 	struct mmu_gather_pending *next;
>>>  }
>>> 
>>> tlb_finish_mmu() would then iterate over the mm->mmu_gather_pending
>>> (pointing to the linked list) and find whether there is any overlap. This
>>> would still require synchronization (acquiring a lock when allocating and
>>> deallocating or something fancier).
>> 
>> We have an interval_tree for this, and yes, that's how far I got :/
>> 
>> 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.

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.

So perhaps just avoiding the batching if only entries from a single table
are flushed would be enough.


  reply	other threads:[~2019-05-13  9:21 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
2019-05-14  8:58                       ` Mel Gorman
2019-05-13  9:12                 ` Peter Zijlstra
2019-05-13  9:21                   ` Nadav Amit [this message]
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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