Re: [PATCH] mm: mmu_gather: remove __tlb_reset_range() for force flush

[Nadav Amit]

[ Restoring the recipients after mistakenly pressing reply instead of
reply-all ]

> On May 9, 2019, at 12:11 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> On Thu, May 09, 2019 at 06:50:00PM +0000, Nadav Amit wrote:
>>> On May 9, 2019, at 11:24 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>>> 
>>> On Thu, May 09, 2019 at 05:36:29PM +0000, Nadav Amit wrote:
> 
>>>> As a simple optimization, I think it is possible to hold multiple nesting
>>>> counters in the mm, similar to tlb_flush_pending, for freed_tables,
>>>> cleared_ptes, etc.
>>>> 
>>>> The first time you set tlb->freed_tables, you also atomically increase
>>>> mm->tlb_flush_freed_tables. Then, in tlb_flush_mmu(), you just use
>>>> mm->tlb_flush_freed_tables instead of tlb->freed_tables.
>>> 
>>> That sounds fraught with races and expensive; I would much prefer to not
>>> go there for this arguably rare case.
>>> 
>>> Consider such fun cases as where CPU-0 sees and clears a PTE, CPU-1
>>> races and doesn't see that PTE. Therefore CPU-0 sets and counts
>>> cleared_ptes. Then if CPU-1 flushes while CPU-0 is still in mmu_gather,
>>> it will see cleared_ptes count increased and flush that granularity,
>>> OTOH if CPU-1 flushes after CPU-0 completes, it will not and potentiall
>>> miss an invalidate it should have had.
>> 
>> CPU-0 would send a TLB shootdown request to CPU-1 when it is done, so I
>> donât see the problem. The TLB shootdown mechanism is independent of the
>> mmu_gather for the matter.
> 
> Duh.. I still don't like those unconditional mm wide atomic counters.
> 
>>> This whole concurrent mmu_gather stuff is horrible.
>>> 
>>> /me ponders more....
>>> 
>>> So I think the fundamental race here is this:
>>> 
>>> 	CPU-0				CPU-1
>>> 
>>> 	tlb_gather_mmu(.start=1,	tlb_gather_mmu(.start=2,
>>> 		       .end=3);			       .end=4);
>>> 
>>> 	ptep_get_and_clear_full(2)
>>> 	tlb_remove_tlb_entry(2);
>>> 	__tlb_remove_page();
>>> 					if (pte_present(2)) // nope
>>> 
>>> 					tlb_finish_mmu();
>>> 
>>> 					// continue without TLBI(2)
>>> 					// whoopsie
>>> 
>>> 	tlb_finish_mmu();
>>> 	  tlb_flush()		->	TLBI(2)
>>> 
>>> 
>>> And we can fix that by having tlb_finish_mmu() sync up. Never let a
>>> concurrent tlb_finish_mmu() complete until all concurrenct mmu_gathers
>>> have completed.
>>> 
>>> This should not be too hard to make happen.
>> 
>> This synchronization sounds much more expensive than what I proposed. But I
>> agree that cache-lines that move from one CPU to another might become an
>> issue. But I think that the scheme I suggested would minimize this overhead.
> 
> Well, it would have a lot more unconditional atomic ops. My scheme only
> waits when there is actual concurrency.

Well, something has to give. I didnât think that if the same core does the
atomic op it would be too expensive.

> I _think_ something like the below ought to work, but its not even been
> near a compiler. The only problem is the unconditional wakeup; we can
> play games to avoid that if we want to continue with this.
> 
> Ideally we'd only do this when there's been actual overlap, but I've not
> found a sensible way to detect that.
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 4ef4bbe78a1d..b70e35792d29 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -590,7 +590,12 @@ static inline void dec_tlb_flush_pending(struct mm_struct *mm)
> 	 *
> 	 * Therefore we must rely on tlb_flush_*() to guarantee order.
> 	 */
> -	atomic_dec(&mm->tlb_flush_pending);
> +	if (atomic_dec_and_test(&mm->tlb_flush_pending)) {
> +		wake_up_var(&mm->tlb_flush_pending);
> +	} else {
> +		wait_event_var(&mm->tlb_flush_pending,
> +			       !atomic_read_acquire(&mm->tlb_flush_pending));
> +	}
> }

It still seems very expensive to me, at least for certain workloads (e.g.,
Apache with multithreaded MPM).

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).