Re: [PATCH v2 5/5] psi: introduce psi monitor

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Jan 16, 2019 at 11:27 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Wed, Jan 16, 2019 at 02:17:28PM -0500, Johannes Weiner wrote:
> > On Wed, Jan 16, 2019 at 09:39:13AM -0800, Suren Baghdasaryan wrote:
> > > On Wed, Jan 16, 2019 at 5:24 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > > >
> > > > On Mon, Jan 14, 2019 at 11:30:12AM -0800, Suren Baghdasaryan wrote:
> > > > > For memory ordering (which Johannes also pointed out) the critical point is:
> > > > >
> > > > > times[cpu] += delta           | if g->polling:
> > > > > smp_wmb()                     |   g->polling = polling = 0
> > > > > cmpxchg(g->polling, 0, 1)     |   smp_rmb()
> > > > >                               |   delta = times[*] (through goto SLOWPATH)
> > > > >
> > > > > So that hotpath writes to times[] then g->polling and slowpath reads
> > > > > g->polling then times[]. cmpxchg() implies a full barrier, so we can
> > > > > drop smp_wmb(). Something like this:
> > > > >
> > > > > times[cpu] += delta           | if g->polling:
> > > > > cmpxchg(g->polling, 0, 1)     |   g->polling = polling = 0
> > > > >                               |   smp_rmb()
> > > > >                               |   delta = times[*] (through goto SLOWPATH)
> > > > >
> > > > > Would that address your concern about ordering?
> > > >
> > > > cmpxchg() implies smp_mb() before and after, so the smp_wmb() on the
> > > > left column is superfluous.
> > >
> > > Should I keep it in the comments to make it obvious and add a note
> > > about implicit barriers being the reason we don't call smp_mb() in the
> > > code explicitly?
> >
> > I'd keep 'em out if they aren't actually in the code. But I'd switch
> >
> >       delta = times[*]
> >
> > in this comment to to
> >
> >       get_recent_times() // implies smp_mb()
>
> Actually, I might have been mistaken about this. The seqcount locking
> does an smp_rmb() and an smp_wmb(), and that orders reads and writes
> respectively, but doesn't necessarily order reads against writes.
>
> So I think we need an explicit smp_mb() after all.

I see. So, the action items I collected so far:

1. Add a comment in the code next to cmpxchg() indicating implicit smp_mb.
2. Add explicit smp_mb after "g->polling = 0" and before "delta =
times[*]" both in the code and in the comments (in the slowpath).
3. Use atomic_t for g->polling. Add a note in the comments why atomic
operations are not needed in the slowpath.
4. Minimize line-breaks.

Please let me know if I missed anything, otherwise will make these
changes and post ver 3 of the patchset.
Thanks,
Suren.