Author: jeff Date: Tue Feb 4 02:44:52 2020 New Revision: 357487 URL: https://svnweb.freebsd.org/changeset/base/357487
Log: Implement a deferred write advancement feature that can be used to further amortize shared cacheline writes. Discussed with: rlibby Differential Revision: https://reviews.freebsd.org/D23462 Modified: head/sys/kern/subr_smr.c head/sys/sys/smr.h Modified: head/sys/kern/subr_smr.c ============================================================================== --- head/sys/kern/subr_smr.c Tue Feb 4 02:42:54 2020 (r357486) +++ head/sys/kern/subr_smr.c Tue Feb 4 02:44:52 2020 (r357487) @@ -209,6 +209,26 @@ smr_advance(smr_t smr) return (goal); } +smr_seq_t +smr_advance_deferred(smr_t smr, int limit) +{ + smr_seq_t goal; + smr_t csmr; + + critical_enter(); + csmr = zpcpu_get(smr); + if (++csmr->c_deferred >= limit) { + goal = SMR_SEQ_INVALID; + csmr->c_deferred = 0; + } else + goal = smr_shared_current(csmr->c_shared) + SMR_SEQ_INCR; + critical_exit(); + if (goal != SMR_SEQ_INVALID) + return (goal); + + return (smr_advance(smr)); +} + /* * Poll to determine whether all readers have observed the 'goal' write * sequence number. @@ -255,6 +275,17 @@ smr_poll(smr_t smr, smr_seq_t goal, bool wait) * c_seq can only reference time after this wr_seq. */ s_wr_seq = atomic_load_acq_int(&s->s_wr_seq); + + /* + * This may have come from a deferred advance. Consider one + * increment past the current wr_seq valid and make sure we + * have advanced far enough to succeed. We simply add to avoid + * an additional fence. + */ + if (goal == s_wr_seq + SMR_SEQ_INCR) { + atomic_add_int(&s->s_wr_seq, SMR_SEQ_INCR); + s_wr_seq = goal; + } /* * Detect whether the goal is valid and has already been observed. Modified: head/sys/sys/smr.h ============================================================================== --- head/sys/sys/smr.h Tue Feb 4 02:42:54 2020 (r357486) +++ head/sys/sys/smr.h Tue Feb 4 02:44:52 2020 (r357487) @@ -64,6 +64,7 @@ typedef struct smr_shared *smr_shared_t; struct smr { smr_seq_t c_seq; /* Current observed sequence. */ smr_shared_t c_shared; /* Shared SMR state. */ + int c_deferred; /* Deferred advance counter. */ }; /* @@ -144,6 +145,13 @@ smr_exit(smr_t smr) * required to ensure that all modifications are visible to readers. */ smr_seq_t smr_advance(smr_t smr); + +/* + * Advances the write sequence number only after N calls. Returns + * the correct goal for a wr_seq that has not yet occurred. Used to + * minimize shared cacheline invalidations for frequent writers. + */ +smr_seq_t smr_advance_deferred(smr_t smr, int limit); /* * Returns true if a goal sequence has been reached. If _______________________________________________ svn-src-all@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-all To unsubscribe, send any mail to "svn-src-all-unsubscr...@freebsd.org"
