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@xxxxxxxxxxxxx> 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.
