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NFSD: Eliminate percpu counter contention in reply cache statistics

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Each RPC passes through nfsd_cache_lookup(), which increments one
of nfsd_stats_rc_hits_inc(), nfsd_stats_rc_misses_inc(), or
nfsd_stats_rc_nocache_inc(). These helpers update
per-net-namespace percpu_counters with percpu_counter_inc(),
which applies the default batch threshold of max(32, 2*nr_cpus).

Once a CPU's local delta reaches that threshold, the update folds
into the shared counter under its global spinlock. On a busy
multi-CPU server this produces lock traffic on a counter cacheline
shared across all CPUs, growing with the request rate.

Switch to percpu_counter_add_local(fbc, 1), which batches with
INT_MAX so that increments always remain on the per-CPU fast path.
This matches the treatment already applied to the IO byte and DRC
memory counters. All readers of these counters use
percpu_counter_sum_positive(), which sums the per-CPU deltas under
the global lock, so read accuracy is unaffected.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: NeilBrown <neil@brown.name>
Link: https://patch.msgid.link/20260717001232.438792-4-cel@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>
This commit is contained in:
Chuck Lever
2026-07-16 20:12:30 -04:00
parent 259dfa726d
commit ee987730d6
1 changed files with 23 additions and 3 deletions
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26
fs/nfsd/stats.h
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@@ -16,19 +16,39 @@
struct proc_dir_entry *nfsd_proc_stat_init(struct net *net);
void nfsd_proc_stat_shutdown(struct net *net);
/**
* nfsd_stats_rc_hits_inc - Count a duplicate reply cache hit
* @nn: target network namespace
*
* These reply cache counters are updated once per RPC. Readers use
* percpu_counter_sum_positive(), so local batching does not affect
* read accuracy.
*/
static inline void nfsd_stats_rc_hits_inc(struct nfsd_net *nn)
{
percpu_counter_inc(&nn->counter[NFSD_STATS_RC_HITS]);
percpu_counter_add_local(&nn->counter[NFSD_STATS_RC_HITS], 1);
}
/**
* nfsd_stats_rc_misses_inc - Count a duplicate reply cache miss
* @nn: target network namespace
*
* See nfsd_stats_rc_hits_inc() for batching rationale.
*/
static inline void nfsd_stats_rc_misses_inc(struct nfsd_net *nn)
{
percpu_counter_inc(&nn->counter[NFSD_STATS_RC_MISSES]);
percpu_counter_add_local(&nn->counter[NFSD_STATS_RC_MISSES], 1);
}
/**
* nfsd_stats_rc_nocache_inc - Count a request not cached in the reply cache
* @nn: target network namespace
*
* See nfsd_stats_rc_hits_inc() for batching rationale.
*/
static inline void nfsd_stats_rc_nocache_inc(struct nfsd_net *nn)
{
percpu_counter_inc(&nn->counter[NFSD_STATS_RC_NOCACHE]);
percpu_counter_add_local(&nn->counter[NFSD_STATS_RC_NOCACHE], 1);
}
static inline void nfsd_stats_fh_stale_inc(struct nfsd_net *nn,