Re: [RFC 15/20] mm: detect deferred TLB flushes in vma granularity

Andy Lutomirski
2021-02-04T04:36:48Z

> On Feb 2, 2021, at 12:52 PM, Nadav Amit <nadav.amit@gmail.com> wrote:
>
> ﻿
>>
>>> On Feb 1, 2021, at 4:14 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>>
>>>
>>>> On Feb 1, 2021, at 2:04 PM, Nadav Amit <nadav.amit@gmail.com> wrote:
>>>
>>> Andy’s comments managed to make me realize this code is wrong. We must
>>> call inc_mm_tlb_gen(mm) every time.
>>>
>>> Otherwise, a CPU that saw the old tlb_gen and updated it in its local
>>> cpu_tlbstate on a context-switch. If the process was not running when the
>>> TLB flush was issued, no IPI will be sent to the CPU. Therefore, later
>>> switch_mm_irqs_off() back to the process will not flush the local TLB.
>>>
>>> I need to think if there is a better solution. Multiple calls to
>>> inc_mm_tlb_gen() during deferred flushes would trigger a full TLB flush
>>> instead of one that is specific to the ranges, once the flush actually takes
>>> place. On x86 it’s practically a non-issue, since anyhow any update of more
>>> than 33-entries or so would cause a full TLB flush, but this is still ugly.
>>
>> What if we had a per-mm ring buffer of flushes? When starting a flush, we would stick the range in the ring buffer and, when flushing, we would read the ring buffer to catch up. This would mostly replace the flush_tlb_info struct, and it would let us process multiple partial flushes together.
>
> I wanted to sleep on it, and went back and forth on whether it is the right
> direction, hence the late response.
>
> I think that what you say make sense. I think that I even tried to do once
> something similar for some reason, but my memory plays tricks on me.
>
> So tell me what you think on this ring-based solution. As you said, you keep
> per-mm ring of flush_tlb_info. When you queue an entry, you do something
> like:
>
> #define RING_ENTRY_INVALID (0)
>
> gen = inc_mm_tlb_gen(mm);
> struct flush_tlb_info *info = mm->ring[gen % RING_SIZE];
> spin_lock(&mm->ring_lock);

Once you are holding the lock, you should presumably check that the ring didn’t overflow while you were getting the lock — if new_tlb_gen > gen, abort.

> WRITE_ONCE(info->new_tlb_gen, RING_ENTRY_INVALID);
> smp_wmb();
> info->start = start;
> info->end = end;
> info->stride_shift = stride_shift;
> info->freed_tables = freed_tables;
> smp_store_release(&info->new_tlb_gen, gen);
> spin_unlock(&mm->ring_lock);
>

Seems reasonable. I’m curious how this ends up getting used.

> When you flush you use the entry generation as a sequence lock. On overflow
> of the ring (i.e., sequence number mismatch) you perform a full flush:
>
> for (gen = mm->tlb_gen_completed; gen < mm->tlb_gen; gen++) {
> struct flush_tlb_info *info = &mm->ring[gen % RING_SIZE];
>
> // detect overflow and invalid entries
> if (smp_load_acquire(info->new_tlb_gen) != gen)
> goto full_flush;
>
> start = min(start, info->start);
> end = max(end, info->end);
> stride_shift = min(stride_shift, info->stride_shift);
> freed_tables |= info.freed_tables;
> smp_rmb();
>
> // seqlock-like check that the information was not updated
> if (READ_ONCE(info->new_tlb_gen) != gen)
> goto full_flush;
> }
>
> On x86 I suspect that performing a full TLB flush would anyhow be the best
> thing to do if there is more than a single entry. I am also not sure that it
> makes sense to check the ring from flush_tlb_func_common() (i.e., in each
> IPI handler) as it might cause cache thrashing.
>
> Instead it may be better to do so from flush_tlb_mm_range(), when the
> flushes are initiated, and use an aggregated flush_tlb_info for the flush.
>
> It may also be better to have the ring arch-independent, so it would
> resemble more of mmu_gather (the parts about the TLB flush information,
> without the freed pages stuff).
>
> We can detect deferred TLB flushes either by storing “deferred_gen” in the
> page-tables/VMA (as I did) or by going over the ring, from tlb_gen_completed
> to tlb_gen, and checking for an overlap. I think page-tables would be most
> efficient/scalable, but perhaps going over the ring would be easier to
> understand logic.
>
> Makes sense? Thoughts?