From: Nicholas Piggin <npiggin@gmail.com>
To: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
Nadav Amit <nadav.amit@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>,
Dave Hansen <dave.hansen@linux.intel.com>,
linux-csky@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
linux-s390@vger.kernel.org, Andy Lutomirski <luto@kernel.org>,
Mel Gorman <mgorman@techsingularity.net>,
Nadav Amit <namit@vmware.com>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
Will Deacon <will@kernel.org>,
x86@kernel.org, Yu Zhao <yuzhao@google.com>
Subject: Re: [RFC 00/20] TLB batching consolidation and enhancements
Date: Sun, 31 Jan 2021 13:30:45 +1000 [thread overview]
Message-ID: <1612063149.2awdsvvmhj.astroid@bobo.none> (raw)
In-Reply-To: <20210131001132.3368247-1-namit@vmware.com>
Excerpts from Nadav Amit's message of January 31, 2021 10:11 am:
> From: Nadav Amit <namit@vmware.com>
>
> There are currently (at least?) 5 different TLB batching schemes in the
> kernel:
>
> 1. Using mmu_gather (e.g., zap_page_range()).
>
> 2. Using {inc|dec}_tlb_flush_pending() to inform other threads on the
> ongoing deferred TLB flush and flushing the entire range eventually
> (e.g., change_protection_range()).
>
> 3. arch_{enter|leave}_lazy_mmu_mode() for sparc and powerpc (and Xen?).
>
> 4. Batching per-table flushes (move_ptes()).
>
> 5. By setting a flag on that a deferred TLB flush operation takes place,
> flushing when (try_to_unmap_one() on x86).
>
> It seems that (1)-(4) can be consolidated. In addition, it seems that
> (5) is racy. It also seems there can be many redundant TLB flushes, and
> potentially TLB-shootdown storms, for instance during batched
> reclamation (using try_to_unmap_one()) if at the same time mmu_gather
> defers TLB flushes.
>
> More aggressive TLB batching may be possible, but this patch-set does
> not add such batching. The proposed changes would enable such batching
> in a later time.
>
> Admittedly, I do not understand how things are not broken today, which
> frightens me to make further batching before getting things in order.
> For instance, why is ok for zap_pte_range() to batch dirty-PTE flushes
> for each page-table (but not in greater granularity). Can't
> ClearPageDirty() be called before the flush, causing writes after
> ClearPageDirty() and before the flush to be lost?
Because it's holding the page table lock which stops page_mkclean from
cleaning the page. Or am I misunderstanding the question?
I'll go through the patches a bit more closely when they all come
through. Sparc and powerpc of course need the arch lazy mode to get
per-page/pte information for operations that are not freeing pages,
which is what mmu gather is designed for.
I wouldn't mind using a similar API so it's less of a black box when
reading generic code, but it might not quite fit the mmu gather API
exactly (most of these paths don't want a full mmu_gather on stack).
>
> This patch-set therefore performs the following changes:
>
> 1. Change mprotect, task_mmu and mapping_dirty_helpers to use mmu_gather
> instead of {inc|dec}_tlb_flush_pending().
>
> 2. Avoid TLB flushes if PTE permission is not demoted.
>
> 3. Cleans up mmu_gather to be less arch-dependant.
>
> 4. Uses mm's generations to track in finer granularity, either per-VMA
> or per page-table, whether a pending mmu_gather operation is
> outstanding. This should allow to avoid some TLB flushes when KSM or
> memory reclamation takes place while another operation such as
> munmap() or mprotect() is running.
>
> 5. Changes try_to_unmap_one() flushing scheme, as the current seems
> broken to track in a bitmap which CPUs have outstanding TLB flushes
> instead of having a flag.
Putting fixes first, and cleanups and independent patches (like #2) next
would help with getting stuff merged and backported.
Thanks,
Nick
next prev parent reply other threads:[~2021-01-31 3:31 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-31 0:11 [RFC 00/20] TLB batching consolidation and enhancements Nadav Amit
2021-01-31 0:11 ` [RFC 11/20] mm/tlb: remove arch-specific tlb_start/end_vma() Nadav Amit
2021-02-01 12:09 ` Peter Zijlstra
2021-02-02 6:41 ` Nicholas Piggin
2021-02-02 7:20 ` Nadav Amit
2021-02-02 9:31 ` Peter Zijlstra
2021-02-02 9:54 ` Nadav Amit
2021-02-02 11:04 ` Peter Zijlstra
2021-01-31 0:39 ` [RFC 00/20] TLB batching consolidation and enhancements Andy Lutomirski
2021-01-31 1:08 ` Nadav Amit
2021-01-31 3:30 ` Nicholas Piggin [this message]
2021-01-31 7:57 ` Nadav Amit
2021-01-31 8:14 ` Nadav Amit
2021-02-01 12:44 ` Peter Zijlstra
2021-02-02 7:14 ` Nicholas Piggin
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