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

From: Matt Fleming
Date: Tue Jul 11 2017 - 07:32:46 EST

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?