Re: [PATCH RFC UGLY] x86,mm,sched: make lazy TLB mode even lazier

Andy Lutomirski
2016-08-28T08:11:45Z

On Aug 25, 2016 9:06 PM, "Rik van Riel" <riel@redhat.com> wrote:
>
> Subject: x86,mm,sched: make lazy TLB mode even lazier
>
> Lazy TLB mode can result in an idle CPU being woken up for a TLB
> flush, when all it really needed to do was flush %cr3 before the
> next context switch.
>
> This is mostly fine on bare metal, though sub-optimal from a power
> saving point of view, and deeper C states could make TLB flushes
> take a little longer than desired.
>
> On virtual machines, the pain can be much worse, especially if a
> currently non-running VCPU is woken up for a TLB invalidation
> IPI, on a CPU that is busy running another task. It could take
> a while before that IPI is handled, leading to performance issues.
>
> This patch is still ugly, and the sched.h include needs to be cleaned
> up a lot (how would the scheduler people like to see the context switch
> blocking abstracted?)
>
> This patch deals with the issue by introducing a third tlb state,
> TLBSTATE_FLUSH, which causes %cr3 to be flushed at the next
> context switch. A CPU is transitioned from TLBSTATE_LAZY to
> TLBSTATE_FLUSH with the rq lock held, to prevent context switches.
>
> Nothing is done for a CPU that is already in TLBSTATE_FLUH mode.
>
> This patch is totally untested, because I am at a conference right
> now, and Benjamin has the test case :)
>

I haven't had a chance to seriously read the code yet, but what
happens when the mm is deleted outright? Or is the idea that a
reference is held until all the lazy users are gone, too?

On PCID systems (still need to get that code upstream...), I wonder if
we could go the other way and stop being lazy, as cr3 writes can be
much faster.

--Andy