Re: [PATCH v4 00/10] PCID and improved laziness

[Andy Lutomirski]

On Tue, Jul 11, 2017 at 4:32 AM, Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx> wrote:
> On Fri, 30 Jun, at 01:44:22PM, Matt Fleming wrote:
>> On Thu, 29 Jun, at 08:53:12AM, Andy Lutomirski wrote:
>> > *** Ingo, even if this misses 4.13, please apply the first patch before
>> > *** the merge window.
>> >
>> > There are three performance benefits here:
>> >
>> > 1. TLB flushing is slow.  (I.e. the flush itself takes a while.)
>> >    This avoids many of them when switching tasks by using PCID.  In
>> >    a stupid little benchmark I did, it saves about 100ns on my laptop
>> >    per context switch.  I'll try to improve that benchmark.
>> >
>> > 2. Mms that have been used recently on a given CPU might get to keep
>> >    their TLB entries alive across process switches with this patch
>> >    set.  TLB fills are pretty fast on modern CPUs, but they're even
>> >    faster when they don't happen.
>> >
>> > 3. Lazy TLB is way better.  We used to do two stupid things when we
>> >    ran kernel threads: we'd send IPIs to flush user contexts on their
>> >    CPUs and then we'd write to CR3 for no particular reason as an excuse
>> >    to stop further IPIs.  With this patch, we do neither.
>>
>> Heads up, I'm gonna queue this for a run on SUSE's performance test
>> grid.
>
> FWIW, I didn't see any change in performance with this series on a
> PCID-capable machine. On the plus side, I didn't see any weird-looking
> bugs either.
>
> Are your benchmarks available anywhere?

https://git.kernel.org/pub/scm/linux/kernel/git/luto/misc-tests.git/

I did:

$ ./context_switch_latency_64 0 process same

and

$ ./madvise_bounce_64 10k [IIRC -- it might have been a different loop count]

--Andy