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
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svn2git
2020-12-20 02:59:44 +00:00
svn path=/head/; revision=357487
@ -209,6 +209,26 @@ smr_advance(smr_t smr)
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return (goal);
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}
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smr_seq_t
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smr_advance_deferred(smr_t smr, int limit)
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{
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smr_seq_t goal;
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smr_t csmr;
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critical_enter();
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csmr = zpcpu_get(smr);
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if (++csmr->c_deferred >= limit) {
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goal = SMR_SEQ_INVALID;
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csmr->c_deferred = 0;
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} else
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goal = smr_shared_current(csmr->c_shared) + SMR_SEQ_INCR;
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critical_exit();
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if (goal != SMR_SEQ_INVALID)
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return (goal);
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return (smr_advance(smr));
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}
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/*
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* Poll to determine whether all readers have observed the 'goal' write
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* sequence number.
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@ -256,6 +276,17 @@ smr_poll(smr_t smr, smr_seq_t goal, bool wait)
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*/
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s_wr_seq = atomic_load_acq_int(&s->s_wr_seq);
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/*
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* This may have come from a deferred advance. Consider one
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* increment past the current wr_seq valid and make sure we
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* have advanced far enough to succeed. We simply add to avoid
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* an additional fence.
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*/
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if (goal == s_wr_seq + SMR_SEQ_INCR) {
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atomic_add_int(&s->s_wr_seq, SMR_SEQ_INCR);
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s_wr_seq = goal;
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}
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/*
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* Detect whether the goal is valid and has already been observed.
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*
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@ -64,6 +64,7 @@ typedef struct smr_shared *smr_shared_t;
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struct smr {
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smr_seq_t c_seq; /* Current observed sequence. */
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smr_shared_t c_shared; /* Shared SMR state. */
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int c_deferred; /* Deferred advance counter. */
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};
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/*
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@ -145,6 +146,13 @@ smr_exit(smr_t smr)
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*/
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smr_seq_t smr_advance(smr_t smr);
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/*
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* Advances the write sequence number only after N calls. Returns
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* the correct goal for a wr_seq that has not yet occurred. Used to
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* minimize shared cacheline invalidations for frequent writers.
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*/
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smr_seq_t smr_advance_deferred(smr_t smr, int limit);
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/*
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* Returns true if a goal sequence has been reached. If
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* wait is true this will busy loop until success.
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