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Code Issues Releases Activity

SCHEDULER_STOPPED(): Rely on a global variable

Browse Source 
A commit from 2012 (5d7380f8e3, r228424) introduced
'td_stopsched', on the ground that a global variable would cause all
CPUs to have a copy of it in their cache, and consequently of all other
variables sharing the same cache line.

This is really a problem only if that cache line sees relatively
frequent modifications.  This was unlikely to be the case back then
because nearby variables are almost never modified as well.  In any
case, today we have a new tool at our disposal to ensure that this
variable goes into a read-mostly section containing frequently-accessed
variables ('__read_frequently').  Most of the cache lines covering this
section are likely to always be in every CPU cache.  This makes the
second reason stated in the commit message (ensuring the field is in the
same cache line as some lock-related fields, since these are accessed in
close proximity) moot, as well as the second order effect of requiring
an additional line to be present in the cache (the one containing the
new 'scheduler_stopped' boolean, see below).

From a pure logical point of view, whether the scheduler is stopped is
a global state and is certainly not a per-thread quality.

Consequently, remove 'td_stopsched', which immediately frees a byte in
'struct thread'.  Currently, the latter's size (and layout) stays
unchanged, but some of the later re-orderings will probably benefit from
this removal.  Available bytes at the original position for
'td_stopsched' have been made explicit with the addition of the
'_td_pad0' member.

Store the global state in the new 'scheduler_stopped' boolean, which is
annotated with '__read_frequently'.

Replace uses of SCHEDULER_STOPPED_TD() with SCHEDULER_STOPPER() and
remove the former as it is now unnecessary.

Reviewed by:            markj, kib
Approved by:            markj (mentor)
MFC after:              2 weeks
Sponsored by:           The FreeBSD Foundation
Differential Revision:  https://reviews.freebsd.org/D43572
This commit is contained in:
Olivier Certner 2024-01-18 14:10:18 +01:00
parent cd0c52e50b
commit 6b35310173
No known key found for this signature in database
GPG Key ID: 8CA13040971E2627
9 changed files with 22 additions and 23 deletions
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10
sys/kern/kern_condvar.c
View File

@ -120,7 +120,7 @@ _cv_wait(struct cv *cvp, struct lock_object *lock)
WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, lock,
"Waiting on \"%s\"", cvp->cv_description);
if (SCHEDULER_STOPPED_TD(td))
if (SCHEDULER_STOPPED())
return;
#ifdef KTRACE
@ -184,7 +184,7 @@ _cv_wait_unlock(struct cv *cvp, struct lock_object *lock)
("cv_wait_unlock cannot be used with Giant"));
class = LOCK_CLASS(lock);
if (SCHEDULER_STOPPED_TD(td)) {
if (SCHEDULER_STOPPED()) {
class->lc_unlock(lock);
return;
}
@ -241,7 +241,7 @@ _cv_wait_sig(struct cv *cvp, struct lock_object *lock)
WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, lock,
"Waiting on \"%s\"", cvp->cv_description);
if (SCHEDULER_STOPPED_TD(td))
if (SCHEDULER_STOPPED())
return (0);
#ifdef KTRACE
@ -309,7 +309,7 @@ _cv_timedwait_sbt(struct cv *cvp, struct lock_object *lock, sbintime_t sbt,
WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, lock,
"Waiting on \"%s\"", cvp->cv_description);
if (SCHEDULER_STOPPED_TD(td))
if (SCHEDULER_STOPPED())
return (0);
#ifdef KTRACE
@ -379,7 +379,7 @@ _cv_timedwait_sig_sbt(struct cv *cvp, struct lock_object *lock,
WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, lock,
"Waiting on \"%s\"", cvp->cv_description);
if (SCHEDULER_STOPPED_TD(td))
if (SCHEDULER_STOPPED())
return (0);
#ifdef KTRACE

4
sys/kern/kern_mutex.c
View File

@ -424,7 +424,7 @@ _mtx_trylock_flags_int(struct mtx *m, int opts LOCK_FILE_LINE_ARG_DEF)
td = curthread;
tid = (uintptr_t)td;
if (SCHEDULER_STOPPED_TD(td))
if (SCHEDULER_STOPPED())
return (1);
KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(td),
@ -534,7 +534,7 @@ __mtx_lock_sleep(volatile uintptr_t *c, uintptr_t v)
doing_lockprof = 1;
#endif
if (SCHEDULER_STOPPED_TD(td))
if (SCHEDULER_STOPPED())
return;
if (__predict_false(v == MTX_UNOWNED))

4
sys/kern/kern_rwlock.c
View File

@ -307,7 +307,7 @@ __rw_try_wlock_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF)
td = curthread;
tid = (uintptr_t)td;
if (SCHEDULER_STOPPED_TD(td))
if (SCHEDULER_STOPPED())
return (1);
KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(td),
@ -666,7 +666,7 @@ __rw_rlock_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF)
td = curthread;
KASSERT(kdb_active != 0 || SCHEDULER_STOPPED_TD(td) ||
KASSERT(kdb_active != 0 || SCHEDULER_STOPPED() ||
!TD_IS_IDLETHREAD(td),
("rw_rlock() by idle thread %p on rwlock %s @ %s:%d",
td, rw->lock_object.lo_name, file, line));

3
sys/kern/kern_shutdown.c
View File

@ -225,6 +225,7 @@ SYSCTL_INT(_kern, OID_AUTO, kerneldump_gzlevel, CTLFLAG_RWTUN,
* to indicate that the kernel has already called panic.
*/
const char *panicstr __read_mostly;
bool scheduler_stopped __read_frequently;
int dumping __read_mostly; /* system is dumping */
int rebooting __read_mostly; /* system is rebooting */
@ -926,7 +927,7 @@ vpanic(const char *fmt, va_list ap)
* Ensure that the scheduler is stopped while panicking, even if panic
* has been entered from kdb.
*/
td->td_stopsched = 1;
scheduler_stopped = true;
bootopt = RB_AUTOBOOT;
newpanic = 0;

2
sys/kern/kern_sx.c
View File

@ -350,7 +350,7 @@ sx_try_xlock_int(struct sx *sx LOCK_FILE_LINE_ARG_DEF)
td = curthread;
tid = (uintptr_t)td;
if (SCHEDULER_STOPPED_TD(td))
if (SCHEDULER_STOPPED())
return (1);
KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(td),

6
sys/kern/kern_synch.c
View File

@ -158,7 +158,7 @@ _sleep(const void *ident, struct lock_object *lock, int priority,
else
class = NULL;
if (SCHEDULER_STOPPED_TD(td)) {
if (SCHEDULER_STOPPED()) {
if (lock != NULL && priority & PDROP)
class->lc_unlock(lock);
return (0);
@ -247,7 +247,7 @@ msleep_spin_sbt(const void *ident, struct mtx *mtx, const char *wmesg,
KASSERT(ident != NULL, ("msleep_spin_sbt: NULL ident"));
KASSERT(TD_IS_RUNNING(td), ("msleep_spin_sbt: curthread not running"));
if (SCHEDULER_STOPPED_TD(td))
if (SCHEDULER_STOPPED())
return (0);
sleepq_lock(ident);
@ -511,7 +511,7 @@ mi_switch(int flags)
*/
if (kdb_active)
kdb_switch();
if (SCHEDULER_STOPPED_TD(td))
if (SCHEDULER_STOPPED())
return;
if (flags & SW_VOL) {
td->td_ru.ru_nvcsw++;

4
sys/kern/subr_kdb.c
View File

@ -764,7 +764,7 @@ kdb_trap(int type, int code, struct trapframe *tf)
CPU_CLR(PCPU_GET(cpuid), &other_cpus);
stop_cpus_hard(other_cpus);
#endif
curthread->td_stopsched = 1;
scheduler_stopped = true;
did_stop_cpus = 1;
} else
did_stop_cpus = 0;
@ -801,7 +801,7 @@ kdb_trap(int type, int code, struct trapframe *tf)
kdb_active--;
if (did_stop_cpus) {
curthread->td_stopsched = 0;
scheduler_stopped = false;
#ifdef SMP
CPU_AND(&other_cpus, &other_cpus, &stopped_cpus);
restart_cpus(other_cpus);

4
sys/sys/proc.h
View File

@ -270,7 +270,7 @@ struct thread {
const char *td_wmesg; /* (t) Reason for sleep. */
volatile u_char td_owepreempt; /* (k*) Preempt on last critical_exit */
u_char td_tsqueue; /* (t) Turnstile queue blocked on. */
u_char td_stopsched; /* (k) Scheduler stopped. */
u_char _td_pad0[2]; /* Available. */
int td_locks; /* (k) Debug: count of non-spin locks */
int td_rw_rlocks; /* (k) Count of rwlock read locks. */
int td_sx_slocks; /* (k) Count of sx shared locks. */
@ -429,7 +429,7 @@ do { \
#define TD_LOCKS_INC(td) ((td)->td_locks++)
#define TD_LOCKS_DEC(td) do { \
KASSERT(SCHEDULER_STOPPED_TD(td) || (td)->td_locks > 0, \
KASSERT(SCHEDULER_STOPPED() || (td)->td_locks > 0, \
("Thread %p owns no locks", (td))); \
(td)->td_locks--; \
} while (0)

8
sys/sys/systm.h
View File

@ -99,17 +99,15 @@ struct ucred;
#include <sys/pcpu.h> /* curthread */
#include <sys/kpilite.h>
extern bool scheduler_stopped;
/*
* If we have already panic'd and this is the thread that called
* panic(), then don't block on any mutexes but silently succeed.
* Otherwise, the kernel will deadlock since the scheduler isn't
* going to run the thread that holds any lock we need.
*/
#define SCHEDULER_STOPPED_TD(td) ({ \
MPASS((td) == curthread); \
__predict_false((td)->td_stopsched); \
})
#define SCHEDULER_STOPPED() SCHEDULER_STOPPED_TD(curthread)
#define SCHEDULER_STOPPED() __predict_false(scheduler_stopped)
extern int osreldate;