Date: Tue, 7 Feb 2017 14:49:36 +0000 (UTC) From: Mateusz Guzik <mjg@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r313386 - in head/sys: kern sys Message-ID: <201702071449.v17EnaUa014301@repo.freebsd.org>
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Author: mjg Date: Tue Feb 7 14:49:36 2017 New Revision: 313386 URL: https://svnweb.freebsd.org/changeset/base/313386 Log: locks: change backoff to exponential Previous implementation would use a random factor to spread readers and reduce chances of starvation. This visibly reduces effectiveness of the mechanism. Switch to the more traditional exponential variant. Try to limit starvation by imposing an upper limit of spins after which spinning is half of what other threads get. Note the mechanism is turned off by default. Reviewed by: kib (previous version) Modified: head/sys/kern/kern_mutex.c head/sys/kern/kern_rwlock.c head/sys/kern/kern_sx.c head/sys/kern/subr_lock.c head/sys/sys/lock.h Modified: head/sys/kern/kern_mutex.c ============================================================================== --- head/sys/kern/kern_mutex.c Tue Feb 7 12:04:04 2017 (r313385) +++ head/sys/kern/kern_mutex.c Tue Feb 7 14:49:36 2017 (r313386) @@ -140,63 +140,27 @@ struct lock_class lock_class_mtx_spin = #ifdef ADAPTIVE_MUTEXES static SYSCTL_NODE(_debug, OID_AUTO, mtx, CTLFLAG_RD, NULL, "mtx debugging"); -static struct lock_delay_config __read_mostly mtx_delay = { - .initial = 1000, - .step = 500, - .min = 100, - .max = 5000, -}; +static struct lock_delay_config __read_mostly mtx_delay; -SYSCTL_INT(_debug_mtx, OID_AUTO, delay_initial, CTLFLAG_RW, &mtx_delay.initial, - 0, ""); -SYSCTL_INT(_debug_mtx, OID_AUTO, delay_step, CTLFLAG_RW, &mtx_delay.step, - 0, ""); -SYSCTL_INT(_debug_mtx, OID_AUTO, delay_min, CTLFLAG_RW, &mtx_delay.min, +SYSCTL_INT(_debug_mtx, OID_AUTO, delay_base, CTLFLAG_RW, &mtx_delay.base, 0, ""); SYSCTL_INT(_debug_mtx, OID_AUTO, delay_max, CTLFLAG_RW, &mtx_delay.max, 0, ""); -static void -mtx_delay_sysinit(void *dummy) -{ - - mtx_delay.initial = mp_ncpus * 25; - mtx_delay.step = (mp_ncpus * 25) / 2; - mtx_delay.min = mp_ncpus * 5; - mtx_delay.max = mp_ncpus * 25 * 10; -} -LOCK_DELAY_SYSINIT(mtx_delay_sysinit); +LOCK_DELAY_SYSINIT_DEFAULT(mtx_delay); #endif static SYSCTL_NODE(_debug, OID_AUTO, mtx_spin, CTLFLAG_RD, NULL, "mtx spin debugging"); -static struct lock_delay_config __read_mostly mtx_spin_delay = { - .initial = 1000, - .step = 500, - .min = 100, - .max = 5000, -}; - -SYSCTL_INT(_debug_mtx_spin, OID_AUTO, delay_initial, CTLFLAG_RW, - &mtx_spin_delay.initial, 0, ""); -SYSCTL_INT(_debug_mtx_spin, OID_AUTO, delay_step, CTLFLAG_RW, &mtx_spin_delay.step, - 0, ""); -SYSCTL_INT(_debug_mtx_spin, OID_AUTO, delay_min, CTLFLAG_RW, &mtx_spin_delay.min, - 0, ""); -SYSCTL_INT(_debug_mtx_spin, OID_AUTO, delay_max, CTLFLAG_RW, &mtx_spin_delay.max, - 0, ""); +static struct lock_delay_config __read_mostly mtx_spin_delay; -static void -mtx_spin_delay_sysinit(void *dummy) -{ +SYSCTL_INT(_debug_mtx_spin, OID_AUTO, delay_base, CTLFLAG_RW, + &mtx_spin_delay.base, 0, ""); +SYSCTL_INT(_debug_mtx_spin, OID_AUTO, delay_max, CTLFLAG_RW, + &mtx_spin_delay.max, 0, ""); - mtx_spin_delay.initial = mp_ncpus * 25; - mtx_spin_delay.step = (mp_ncpus * 25) / 2; - mtx_spin_delay.min = mp_ncpus * 5; - mtx_spin_delay.max = mp_ncpus * 25 * 10; -} -LOCK_DELAY_SYSINIT(mtx_spin_delay_sysinit); +LOCK_DELAY_SYSINIT_DEFAULT(mtx_spin_delay); /* * System-wide mutexes Modified: head/sys/kern/kern_rwlock.c ============================================================================== --- head/sys/kern/kern_rwlock.c Tue Feb 7 12:04:04 2017 (r313385) +++ head/sys/kern/kern_rwlock.c Tue Feb 7 14:49:36 2017 (r313386) @@ -100,32 +100,14 @@ static SYSCTL_NODE(_debug, OID_AUTO, rwl SYSCTL_INT(_debug_rwlock, OID_AUTO, retry, CTLFLAG_RW, &rowner_retries, 0, ""); SYSCTL_INT(_debug_rwlock, OID_AUTO, loops, CTLFLAG_RW, &rowner_loops, 0, ""); -static struct lock_delay_config __read_mostly rw_delay = { - .initial = 1000, - .step = 500, - .min = 100, - .max = 5000, -}; +static struct lock_delay_config __read_mostly rw_delay; -SYSCTL_INT(_debug_rwlock, OID_AUTO, delay_initial, CTLFLAG_RW, &rw_delay.initial, - 0, ""); -SYSCTL_INT(_debug_rwlock, OID_AUTO, delay_step, CTLFLAG_RW, &rw_delay.step, - 0, ""); -SYSCTL_INT(_debug_rwlock, OID_AUTO, delay_min, CTLFLAG_RW, &rw_delay.min, +SYSCTL_INT(_debug_rwlock, OID_AUTO, delay_base, CTLFLAG_RW, &rw_delay.base, 0, ""); SYSCTL_INT(_debug_rwlock, OID_AUTO, delay_max, CTLFLAG_RW, &rw_delay.max, 0, ""); -static void -rw_delay_sysinit(void *dummy) -{ - - rw_delay.initial = mp_ncpus * 25; - rw_delay.step = (mp_ncpus * 25) / 2; - rw_delay.min = mp_ncpus * 5; - rw_delay.max = mp_ncpus * 25 * 10; -} -LOCK_DELAY_SYSINIT(rw_delay_sysinit); +LOCK_DELAY_SYSINIT_DEFAULT(rw_delay); #endif /* Modified: head/sys/kern/kern_sx.c ============================================================================== --- head/sys/kern/kern_sx.c Tue Feb 7 12:04:04 2017 (r313385) +++ head/sys/kern/kern_sx.c Tue Feb 7 14:49:36 2017 (r313386) @@ -148,32 +148,14 @@ static SYSCTL_NODE(_debug, OID_AUTO, sx, SYSCTL_UINT(_debug_sx, OID_AUTO, retries, CTLFLAG_RW, &asx_retries, 0, ""); SYSCTL_UINT(_debug_sx, OID_AUTO, loops, CTLFLAG_RW, &asx_loops, 0, ""); -static struct lock_delay_config __read_mostly sx_delay = { - .initial = 1000, - .step = 500, - .min = 100, - .max = 5000, -}; +static struct lock_delay_config __read_mostly sx_delay; -SYSCTL_INT(_debug_sx, OID_AUTO, delay_initial, CTLFLAG_RW, &sx_delay.initial, - 0, ""); -SYSCTL_INT(_debug_sx, OID_AUTO, delay_step, CTLFLAG_RW, &sx_delay.step, - 0, ""); -SYSCTL_INT(_debug_sx, OID_AUTO, delay_min, CTLFLAG_RW, &sx_delay.min, +SYSCTL_INT(_debug_sx, OID_AUTO, delay_base, CTLFLAG_RW, &sx_delay.base, 0, ""); SYSCTL_INT(_debug_sx, OID_AUTO, delay_max, CTLFLAG_RW, &sx_delay.max, 0, ""); -static void -sx_delay_sysinit(void *dummy) -{ - - sx_delay.initial = mp_ncpus * 25; - sx_delay.step = (mp_ncpus * 25) / 2; - sx_delay.min = mp_ncpus * 5; - sx_delay.max = mp_ncpus * 25 * 10; -} -LOCK_DELAY_SYSINIT(sx_delay_sysinit); +LOCK_DELAY_SYSINIT_DEFAULT(sx_delay); #endif void Modified: head/sys/kern/subr_lock.c ============================================================================== --- head/sys/kern/subr_lock.c Tue Feb 7 12:04:04 2017 (r313385) +++ head/sys/kern/subr_lock.c Tue Feb 7 14:49:36 2017 (r313386) @@ -56,6 +56,9 @@ __FBSDID("$FreeBSD$"); #include <machine/cpufunc.h> +SDT_PROVIDER_DEFINE(lock); +SDT_PROBE_DEFINE1(lock, , , starvation, "u_int"); + CTASSERT(LOCK_CLASS_MAX == 15); struct lock_class *lock_classes[LOCK_CLASS_MAX + 1] = { @@ -103,32 +106,56 @@ lock_destroy(struct lock_object *lock) lock->lo_flags &= ~LO_INITIALIZED; } +static SYSCTL_NODE(_debug, OID_AUTO, lock, CTLFLAG_RD, NULL, "lock debugging"); +static SYSCTL_NODE(_debug_lock, OID_AUTO, delay, CTLFLAG_RD, NULL, + "lock delay"); + +static u_int __read_mostly starvation_limit = 131072; +SYSCTL_INT(_debug_lock_delay, OID_AUTO, starvation_limit, CTLFLAG_RW, + &starvation_limit, 0, ""); + +static u_int __read_mostly restrict_starvation = 0; +SYSCTL_INT(_debug_lock_delay, OID_AUTO, restrict_starvation, CTLFLAG_RW, + &restrict_starvation, 0, ""); + void lock_delay(struct lock_delay_arg *la) { - u_int i, delay, backoff, min, max; struct lock_delay_config *lc = la->config; + u_int i; - delay = la->delay; + la->delay <<= 1; + if (__predict_false(la->delay > lc->max)) + la->delay = lc->max; - if (delay == 0) - delay = lc->initial; - else { - delay += lc->step; - max = lc->max; - if (delay > max) - delay = max; - } - - backoff = cpu_ticks() % delay; - min = lc->min; - if (backoff < min) - backoff = min; - for (i = 0; i < backoff; i++) + for (i = la->delay; i > 0; i++) cpu_spinwait(); - la->delay = delay; - la->spin_cnt += backoff; + la->spin_cnt += la->delay; + if (__predict_false(la->spin_cnt > starvation_limit)) { + SDT_PROBE1(lock, , , starvation, la->delay); + if (restrict_starvation) + la->delay = lc->base; + } +} + +static u_int +lock_roundup_2(u_int val) +{ + u_int res; + + for (res = 1; res <= val; res <<= 1) + continue; + + return (res); +} + +void +lock_delay_default_init(struct lock_delay_config *lc) +{ + + lc->base = lock_roundup_2(mp_ncpus) / 4; + lc->max = lc->base * 1024; } #ifdef DDB @@ -655,7 +682,6 @@ out: critical_exit(); } -static SYSCTL_NODE(_debug, OID_AUTO, lock, CTLFLAG_RD, NULL, "lock debugging"); static SYSCTL_NODE(_debug_lock, OID_AUTO, prof, CTLFLAG_RD, NULL, "lock profiling"); SYSCTL_INT(_debug_lock_prof, OID_AUTO, skipspin, CTLFLAG_RW, Modified: head/sys/sys/lock.h ============================================================================== --- head/sys/sys/lock.h Tue Feb 7 12:04:04 2017 (r313385) +++ head/sys/sys/lock.h Tue Feb 7 14:49:36 2017 (r313386) @@ -202,9 +202,7 @@ extern struct lock_class lock_class_lock extern struct lock_class *lock_classes[]; struct lock_delay_config { - u_int initial; - u_int step; - u_int min; + u_int base; u_int max; }; @@ -215,19 +213,25 @@ struct lock_delay_arg { }; static inline void -lock_delay_arg_init(struct lock_delay_arg *la, struct lock_delay_config *lc) { +lock_delay_arg_init(struct lock_delay_arg *la, struct lock_delay_config *lc) +{ la->config = lc; - la->delay = 0; + la->delay = lc->base; la->spin_cnt = 0; } #define LOCK_DELAY_SYSINIT(func) \ SYSINIT(func##_ld, SI_SUB_LOCK, SI_ORDER_ANY, func, NULL) +#define LOCK_DELAY_SYSINIT_DEFAULT(lc) \ + SYSINIT(lock_delay_##lc##_ld, SI_SUB_LOCK, SI_ORDER_ANY, \ + lock_delay_default_init, &lc) + void lock_init(struct lock_object *, struct lock_class *, const char *, const char *, int); void lock_destroy(struct lock_object *); void lock_delay(struct lock_delay_arg *); +void lock_delay_default_init(struct lock_delay_config *); void spinlock_enter(void); void spinlock_exit(void); void witness_init(struct lock_object *, const char *);
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