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

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Jan 14, 2019 at 11:30:12AM -0800, Suren Baghdasaryan wrote:
> On Mon, Jan 14, 2019 at 2:22 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Thu, Jan 10, 2019 at 02:07:18PM -0800, Suren Baghdasaryan wrote:
> > > +/*
> > > + * psi_update_work represents slowpath accounting part while
> > > + * psi_group_change represents hotpath part.
> > > + * There are two potential races between these path:
> > > + * 1. Changes to group->polling when slowpath checks for new stall, then
> > > + *    hotpath records new stall and then slowpath resets group->polling
> > > + *    flag. This leads to the exit from the polling mode while monitored
> > > + *    states are still changing.
> > > + * 2. Slowpath overwriting an immediate update scheduled from the hotpath
> > > + *    with a regular update further in the future and missing the
> > > + *    immediate update.
> > > + * Both races are handled with a retry cycle in the slowpath:
> > > + *
> > > + *    HOTPATH:                         |    SLOWPATH:
> > > + *                                     |
> > > + * A) times[cpu] += delta              | E) delta = times[*]
> > > + * B) start_poll = (delta[poll_mask] &&|    if delta[poll_mask]:
> > > + *      cmpxchg(g->polling, 0, 1) == 0)| F)   polling_until = now +
> > > + *                                     |              grace_period
> > > + *                                     |    if now > polling_until:
> > > + *    if start_poll:                   |      if g->polling:
> > > + * C)   mod_delayed_work(1)            | G)     g->polling = polling = 0
> > > + *    else if !delayed_work_pending(): | H)     goto SLOWPATH
> > > + * D)   schedule_delayed_work(PSI_FREQ)|    else:
> > > + *                                     |      if !g->polling:
> > > + *                                     | I)     g->polling = polling = 1
> > > + *                                     | J) if delta && first_pass:
> > > + *                                     |      next_avg = calculate_averages()
> > > + *                                     |      if polling:
> > > + *                                     |        next_poll = poll_triggers()
> > > + *                                     |    if (delta && first_pass) || polling:
> > > + *                                     | K)   mod_delayed_work(
> > > + *                                     |          min(next_avg, next_poll))
> > > + *                                     |      if !polling:
> > > + *                                     |        first_pass = false
> > > + *                                     | L)     goto SLOWPATH
> > > + *
> > > + * Race #1 is represented by (EABGD) sequence in which case slowpath
> > > + * deactivates polling mode because it misses new monitored stall and hotpath
> > > + * doesn't activate it because at (B) g->polling is not yet reset by slowpath
> > > + * in (G). This race is handled by the (H) retry, which in the race described
> > > + * above results in the new sequence of (EABGDHEIK) that reactivates polling
> > > + * mode.
> > > + *
> > > + * Race #2 is represented by polling==false && (JABCK) sequence which
> > > + * overwrites immediate update scheduled at (C) with a later (next_avg) update
> > > + * scheduled at (K). This race is handled by the (L) retry which results in the
> > > + * new sequence of polling==false && (JABCKLEIK) that reactivates polling mode
> > > + * and reschedules next polling update (next_poll).
> > > + *
> > > + * Note that retries can't result in an infinite loop because retry #1 happens
> > > + * only during polling reactivation and retry #2 happens only on the first
> > > + * pass. Constant reactivations are impossible because polling will stay active
> > > + * for at least grace_period. Worst case scenario involves two retries (HEJKLE)
> > > + */
> >
> > I'm having a fairly hard time with this. There's a distinct lack of
> > memory ordering, and a suspicious mixing of atomic ops (cmpxchg) and
> > regular loads and stores (without READ_ONCE/WRITE_ONCE even).
> >
> > Please clarify.
> 
> Thanks for the feedback.
> I do mix atomic and regular loads with g->polling only because the
> slowpath is the only one that resets it back to 0, so
> cmpxchg(g->polling, 1, 0) == 1 at (G) would always return 1.
> Setting g->polling back to 1 at (I) indeed needs an atomic operation
> but at that point it does not matter whether hotpath or slowpath sets
> it. In either case we will schedule a polling update.
> Am I missing anything?
> 
> 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)

delta = times[*] is get_recent_times(), which uses a seqcount and so
implies the smp_rmb() already as well. So we shouldn't need another
explicit one. But the comment should point out all the barriers.