From: Andy Lutomirski <luto@kernel.org>
To: Nadav Amit <namit@vmware.com>
Cc: Andy Lutomirski <luto@kernel.org>, Linux-MM <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>,
Andrea Arcangeli <aarcange@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>,
Dave Hansen <dave.hansen@linux.intel.com>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
Will Deacon <will@kernel.org>, Yu Zhao <yuzhao@google.com>,
Nick Piggin <npiggin@gmail.com>, X86 ML <x86@kernel.org>
Subject: Re: [RFC 08/20] mm: store completed TLB generation
Date: Mon, 1 Feb 2021 08:53:49 -0800 [thread overview]
Message-ID: <CALCETrXZVxqKqryi0xJ+qTLQgHx6XeF_MGbL3qi_5MzygFTJSA@mail.gmail.com> (raw)
In-Reply-To: <17B9FE7B-1C92-4890-9B04-B129178C1184@vmware.com>
On Sun, Jan 31, 2021 at 11:28 PM Nadav Amit <namit@vmware.com> wrote:
>
> > On Jan 31, 2021, at 12:32 PM, Andy Lutomirski <luto@kernel.org> wrote:
> >
> > On Sat, Jan 30, 2021 at 4:16 PM Nadav Amit <nadav.amit@gmail.com> wrote:
> >> From: Nadav Amit <namit@vmware.com>
> >>
> >> To detect deferred TLB flushes in fine granularity, we need to keep
> >> track on the completed TLB flush generation for each mm.
> >>
> >> Add logic to track for each mm the tlb_gen_completed, which tracks the
> >> completed TLB generation. It is the arch responsibility to call
> >> mark_mm_tlb_gen_done() whenever a TLB flush is completed.
> >>
> >> Start the generation numbers from 1 instead of 0. This would allow later
> >> to detect whether flushes of a certain generation were completed.
> >
> > Can you elaborate on how this helps?
>
> I guess it should have gone to patch 15.
>
> The relevant code it interacts with is in read_defer_tlb_flush_gen(). It
> allows to use a single check to see “outdated” deferred TLB gen. Initially
> tlb->defer_gen is zero. We are going to do inc_mm_tlb_gen() both on the
> first time we defer TLB entries and whenever we see mm_gen is newer than
> tlb->defer_gen:
>
> + mm_gen = atomic64_read(&mm->tlb_gen);
> +
> + /*
> + * This condition checks for both first deferred TLB flush and for other
> + * TLB pending or executed TLB flushes after the last table that we
> + * updated. In the latter case, we are going to skip a generation, which
> + * would lead to a full TLB flush. This should therefore not cause
> + * correctness issues, and should not induce overheads, since anyhow in
> + * TLB storms it is better to perform full TLB flush.
> + */
> + if (mm_gen != tlb->defer_gen) {
> + VM_BUG_ON(mm_gen < tlb->defer_gen);
> +
> + tlb->defer_gen = inc_mm_tlb_gen(mm);
> + }
>
>
> >
> > I think you should document that tlb_gen_completed only means that no
> > outdated TLB entries will be observably used. In the x86
> > implementation it's possible for older TLB entries to still exist,
> > unused, in TLBs of cpus running other mms.
>
> You mean entries that be later flushed during switch_mm_irqs_off(), right? I
> think that overall my comments need some work. Yes.
That's exactly what I mean.
>
> > How does this work with arch_tlbbatch_flush()?
>
> completed_gen is not updated by arch_tlbbatch_flush(), since I couldn’t find
> a way to combine them. completed_gen might not catch up with tlb_gen in this
> case until another TLB flush takes place. I do not see correctness issue,
> but it might result in redundant TLB flush.
Please at least document this.
FWIW, arch_tlbbatch_flush() is gross. I'm not convinced it's really
supportable with proper broadcast invalidation. I suppose we could
remove it or explicitly track the set of mms that need flushing.
>
> >> Signed-off-by: Nadav Amit <namit@vmware.com>
> >> Cc: Andrea Arcangeli <aarcange@redhat.com>
> >> Cc: Andrew Morton <akpm@linux-foundation.org>
> >> Cc: Andy Lutomirski <luto@kernel.org>
> >> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> >> Cc: Peter Zijlstra <peterz@infradead.org>
> >> Cc: Thomas Gleixner <tglx@linutronix.de>
> >> Cc: Will Deacon <will@kernel.org>
> >> Cc: Yu Zhao <yuzhao@google.com>
> >> Cc: Nick Piggin <npiggin@gmail.com>
> >> Cc: x86@kernel.org
> >> ---
> >> arch/x86/mm/tlb.c | 10 ++++++++++
> >> include/asm-generic/tlb.h | 33 +++++++++++++++++++++++++++++++++
> >> include/linux/mm_types.h | 15 ++++++++++++++-
> >> 3 files changed, 57 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> >> index 7ab21430be41..d17b5575531e 100644
> >> --- a/arch/x86/mm/tlb.c
> >> +++ b/arch/x86/mm/tlb.c
> >> @@ -14,6 +14,7 @@
> >> #include <asm/nospec-branch.h>
> >> #include <asm/cache.h>
> >> #include <asm/apic.h>
> >> +#include <asm/tlb.h>
> >>
> >> #include "mm_internal.h"
> >>
> >> @@ -915,6 +916,9 @@ void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
> >> if (cpumask_any_but(mm_cpumask(mm), cpu) < nr_cpu_ids)
> >> flush_tlb_others(mm_cpumask(mm), info);
> >>
> >> + /* Update the completed generation */
> >> + mark_mm_tlb_gen_done(mm, new_tlb_gen);
> >> +
> >> put_flush_tlb_info();
> >> put_cpu();
> >> }
> >> @@ -1147,6 +1151,12 @@ void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch)
> >>
> >> cpumask_clear(&batch->cpumask);
> >>
> >> + /*
> >> + * We cannot call mark_mm_tlb_gen_done() since we do not know which
> >> + * mm's should be flushed. This may lead to some unwarranted TLB
> >> + * flushes, but not to correction problems.
> >> + */
> >> +
> >> put_cpu();
> >> }
> >>
> >> diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
> >> index 517c89398c83..427bfcc6cdec 100644
> >> --- a/include/asm-generic/tlb.h
> >> +++ b/include/asm-generic/tlb.h
> >> @@ -513,6 +513,39 @@ static inline void tlb_end_vma(struct mmu_gather *tlb, struct vm_area_struct *vm
> >> }
> >> #endif
> >>
> >> +#ifdef CONFIG_ARCH_HAS_TLB_GENERATIONS
> >> +
> >> +/*
> >> + * Helper function to update a generation to have a new value, as long as new
> >> + * value is greater or equal to gen.
> >> + */
> >
> > I read this a couple of times, and I don't understand it. How about:
> >
> > Helper function to atomically set *gen = max(*gen, new_gen)
> >
> >> +static inline void tlb_update_generation(atomic64_t *gen, u64 new_gen)
> >> +{
> >> + u64 cur_gen = atomic64_read(gen);
> >> +
> >> + while (cur_gen < new_gen) {
> >> + u64 old_gen = atomic64_cmpxchg(gen, cur_gen, new_gen);
> >> +
> >> + /* Check if we succeeded in the cmpxchg */
> >> + if (likely(cur_gen == old_gen))
> >> + break;
> >> +
> >> + cur_gen = old_gen;
> >> + };
> >> +}
> >> +
> >> +
> >> +static inline void mark_mm_tlb_gen_done(struct mm_struct *mm, u64 gen)
> >> +{
> >> + /*
> >> + * Update the completed generation to the new generation if the new
> >> + * generation is greater than the previous one.
> >> + */
> >> + tlb_update_generation(&mm->tlb_gen_completed, gen);
> >> +}
> >> +
> >> +#endif /* CONFIG_ARCH_HAS_TLB_GENERATIONS */
> >> +
> >> /*
> >> * tlb_flush_{pte|pmd|pud|p4d}_range() adjust the tlb->start and tlb->end,
> >> * and set corresponding cleared_*.
> >> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> >> index 2035ac319c2b..8a5eb4bfac59 100644
> >> --- a/include/linux/mm_types.h
> >> +++ b/include/linux/mm_types.h
> >> @@ -571,6 +571,13 @@ struct mm_struct {
> >> * This is not used on Xen PV.
> >> */
> >> atomic64_t tlb_gen;
> >> +
> >> + /*
> >> + * TLB generation which is guarnateed to be flushed, including
> >
> > guaranteed
> >
> >> + * all the PTE changes that were performed before tlb_gen was
> >> + * incremented.
> >> + */
> >
> > I will defer judgment to future patches before I believe that this isn't racy :)
>
> Fair enough. Thanks for the review.
>
next prev parent reply other threads:[~2021-02-01 16:54 UTC|newest]
Thread overview: 67+ 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 01/20] mm/tlb: fix fullmm semantics Nadav Amit
2021-01-31 1:02 ` Andy Lutomirski
2021-01-31 1:19 ` Nadav Amit
2021-01-31 2:57 ` Andy Lutomirski
2021-02-01 7:30 ` Nadav Amit
2021-02-01 11:36 ` Peter Zijlstra
2021-02-02 9:32 ` Nadav Amit
2021-02-02 11:00 ` Peter Zijlstra
2021-02-02 21:35 ` Nadav Amit
2021-02-03 9:44 ` Will Deacon
2021-02-04 3:20 ` Nadav Amit
2021-01-31 0:11 ` [RFC 02/20] mm/mprotect: use mmu_gather Nadav Amit
2021-01-31 0:11 ` [RFC 03/20] mm/mprotect: do not flush on permission promotion Nadav Amit
2021-01-31 1:07 ` Andy Lutomirski
2021-01-31 1:17 ` Nadav Amit
2021-01-31 2:59 ` Andy Lutomirski
[not found] ` <7a6de15a-a570-31f2-14d6-a8010296e694@citrix.com>
2021-02-01 5:58 ` Nadav Amit
2021-02-01 15:38 ` Andrew Cooper
2021-01-31 0:11 ` [RFC 04/20] mm/mapping_dirty_helpers: use mmu_gather Nadav Amit
2021-01-31 0:11 ` [RFC 05/20] mm/tlb: move BATCHED_UNMAP_TLB_FLUSH to tlb.h Nadav Amit
2021-01-31 0:11 ` [RFC 06/20] fs/task_mmu: use mmu_gather interface of clear-soft-dirty Nadav Amit
2021-01-31 0:11 ` [RFC 07/20] mm: move x86 tlb_gen to generic code Nadav Amit
2021-01-31 18:26 ` Andy Lutomirski
2021-01-31 0:11 ` [RFC 08/20] mm: store completed TLB generation Nadav Amit
2021-01-31 20:32 ` Andy Lutomirski
2021-02-01 7:28 ` Nadav Amit
2021-02-01 16:53 ` Andy Lutomirski [this message]
2021-02-01 11:52 ` Peter Zijlstra
2021-01-31 0:11 ` [RFC 09/20] mm: create pte/pmd_tlb_flush_pending() Nadav Amit
2021-01-31 0:11 ` [RFC 10/20] mm: add pte_to_page() 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 0:11 ` [RFC 12/20] mm/tlb: save the VMA that is flushed during tlb_start_vma() Nadav Amit
2021-02-01 12:28 ` Peter Zijlstra
2021-01-31 0:11 ` [RFC 13/20] mm/tlb: introduce tlb_start_ptes() and tlb_end_ptes() Nadav Amit
2021-01-31 9:57 ` Damian Tometzki
2021-01-31 10:07 ` Damian Tometzki
2021-02-01 7:29 ` Nadav Amit
2021-02-01 13:19 ` Peter Zijlstra
2021-02-01 23:00 ` Nadav Amit
2021-01-31 0:11 ` [RFC 14/20] mm: move inc/dec_tlb_flush_pending() to mmu_gather.c Nadav Amit
2021-01-31 0:11 ` [RFC 15/20] mm: detect deferred TLB flushes in vma granularity Nadav Amit
2021-02-01 22:04 ` Nadav Amit
2021-02-02 0:14 ` Andy Lutomirski
2021-02-02 20:51 ` Nadav Amit
2021-02-04 4:35 ` Andy Lutomirski
2021-01-31 0:11 ` [RFC 16/20] mm/tlb: per-page table generation tracking Nadav Amit
2021-01-31 0:11 ` [RFC 17/20] mm/tlb: updated completed deferred TLB flush conditionally Nadav Amit
2021-01-31 0:11 ` [RFC 18/20] mm: make mm_cpumask() volatile Nadav Amit
2021-01-31 0:11 ` [RFC 19/20] lib/cpumask: introduce cpumask_atomic_or() Nadav Amit
2021-01-31 0:11 ` [RFC 20/20] mm/rmap: avoid potential races Nadav Amit
2021-08-23 8:05 ` Huang, Ying
2021-08-23 15:50 ` Nadav Amit
2021-08-24 0:36 ` Huang, Ying
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
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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