Re: [RFC]x86: clearing access bit don't flush tlb

[Shaohua Li]

On Tue, Jan 08, 2013 at 02:03:25AM -0500, Rik van Riel wrote:
> On 01/08/2013 12:09 AM, H. Peter Anvin wrote:
> >On 01/07/2013 09:08 PM, Rik van Riel wrote:
> >>On 01/08/2013 12:03 AM, H. Peter Anvin wrote:
> >>>On 01/07/2013 08:55 PM, Shaohua Li wrote:
> >>>>
> >>>>I searched a little bit, the change (doing TLB flush to clear access
> >>>>bit) is
> >>>>made between 2.6.7 - 2.6.8, I can't find the changelog, but I found a
> >>>>patch:
> >>>>http://www.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.7-rc2/2.6.7-rc2-mm2/broken-out/mm-flush-tlb-when-clearing-young.patch
> >>>>
> >>>>
> >>>>The changelog declaims this is for arm/ppc/ppc64.
> >>>>
> >>>
> >>>Not really.  It says that those have stumbled over it already.  It is
> >>>true in general that this change will make very frequently used pages
> >>>(which stick in the TLB) candidates for eviction.
> >>
> >>That is only true if the pages were to stay in the TLB for a
> >>very very long time.  Probably multiple seconds.
> >>
> >>>x86 would seem to be just as affected, although possibly with a
> >>>different frequency.
> >>>
> >>>Do we have any actual metrics on anything here?
> >>
> >>I suspect that if we do need to force a TLB flush for page
> >>reclaim purposes, it may make sense to do that TLB flush
> >>asynchronously. For example, kswapd could kick off a TLB
> >>flush of every CPU in the system once a second, when the
> >>system is under pageout pressure.
> >>
> >>We would have to do this in a smart way, so the kswapds
> >>from multiple nodes do not duplicate the work.
> >>
> >>If people want that kind of functionality, I would be
> >>happy to cook up an RFC patch.
> >>
> >
> >So it sounds like you're saying that this patch should never have been
> >applied in the first place?
> 
> It made sense at the time.

So you agreed the patch is safe, right?
 
> However, with larger SMP systems, we may need a different
> mechanism to get the TLB flushes done after we clear a bunch
> of accessed bits.
> 
> One thing we could do is mark bits in a bitmap, keeping track
> of which CPUs should have their TLB flushed due to accessed bit
> scanning.
> 
> Then we could set a timer for eg. a 1 second timeout, after
> which the TLB flush IPIs get sent. If the timer is already
> pending, we do not start it, but piggyback on the invocation
> that is already scheduled to happen.
> 
> Does something like that make sense?

I don't understand why larger SMP system matters here. Only if there are enough
TLB entries in CPU matters to me. And if the system is larger, memory is
larger. TLB entries will not be sufficient. Or you are worrying about future
larger SMP system can have very big TLB entries?

Thanks,
Shaohua
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