svn commit: r349326 - head/sys/amd64/amd64
Konstantin Belousov
kib at FreeBSD.org
Sun Jun 23 21:21:12 UTC 2019
Author: kib
Date: Sun Jun 23 21:21:11 2019
New Revision: 349326
URL: https://svnweb.freebsd.org/changeset/base/349326
Log:
amd64 pmap: block on turnstile for lock-less DI.
Port the code to block on turnstile instead of yielding, to lock-less
delayed invalidation. The yield might cause tight loop due to priority
inversion.
Since it is impossible to avoid race between block and wake-up, arm
1-tick callout to wakeup when thread blocks itself.
Reported and tested by: mjg
Reviewed by: alc, markj
Sponsored by: The FreeBSD Foundation
MFC after: 2 months
Differential revision: https://reviews.freebsd.org/D20636
Modified:
head/sys/amd64/amd64/pmap.c
Modified: head/sys/amd64/amd64/pmap.c
==============================================================================
--- head/sys/amd64/amd64/pmap.c Sun Jun 23 21:17:41 2019 (r349325)
+++ head/sys/amd64/amd64/pmap.c Sun Jun 23 21:21:11 2019 (r349326)
@@ -484,6 +484,9 @@ static struct pmap_invl_gen pmap_invl_gen_head = {
.next = NULL,
};
static u_long pmap_invl_gen = 1;
+static int pmap_invl_waiters;
+static struct callout pmap_invl_callout;
+static bool pmap_invl_callout_inited;
#define PMAP_ASSERT_NOT_IN_DI() \
KASSERT(pmap_not_in_di(), ("DI already started"))
@@ -538,6 +541,34 @@ pmap_thread_init_invl_gen_l(struct thread *td)
invl_gen->gen = 0;
}
+static void
+pmap_delayed_invl_wait_block(u_long *m_gen, u_long *invl_gen)
+{
+ struct turnstile *ts;
+
+ ts = turnstile_trywait(&invl_gen_ts);
+ if (*m_gen > atomic_load_long(invl_gen))
+ turnstile_wait(ts, NULL, TS_SHARED_QUEUE);
+ else
+ turnstile_cancel(ts);
+}
+
+static void
+pmap_delayed_invl_finish_unblock(u_long new_gen)
+{
+ struct turnstile *ts;
+
+ turnstile_chain_lock(&invl_gen_ts);
+ ts = turnstile_lookup(&invl_gen_ts);
+ if (new_gen != 0)
+ pmap_invl_gen = new_gen;
+ if (ts != NULL) {
+ turnstile_broadcast(ts, TS_SHARED_QUEUE);
+ turnstile_unpend(ts);
+ }
+ turnstile_chain_unlock(&invl_gen_ts);
+}
+
/*
* Start a new Delayed Invalidation (DI) block of code, executed by
* the current thread. Within a DI block, the current thread may
@@ -582,24 +613,15 @@ static void
pmap_delayed_invl_finish_l(void)
{
struct pmap_invl_gen *invl_gen, *next;
- struct turnstile *ts;
invl_gen = &curthread->td_md.md_invl_gen;
KASSERT(invl_gen->gen != 0, ("missed invl_start"));
mtx_lock(&invl_gen_mtx);
next = LIST_NEXT(invl_gen, link);
- if (next == NULL) {
- turnstile_chain_lock(&invl_gen_ts);
- ts = turnstile_lookup(&invl_gen_ts);
- pmap_invl_gen = invl_gen->gen;
- if (ts != NULL) {
- turnstile_broadcast(ts, TS_SHARED_QUEUE);
- turnstile_unpend(ts);
- }
- turnstile_chain_unlock(&invl_gen_ts);
- } else {
+ if (next == NULL)
+ pmap_delayed_invl_finish_unblock(invl_gen->gen);
+ else
next->gen = invl_gen->gen;
- }
LIST_REMOVE(invl_gen, link);
mtx_unlock(&invl_gen_mtx);
invl_gen->gen = 0;
@@ -856,6 +878,8 @@ again:
goto again;
}
critical_exit();
+ if (atomic_load_int(&pmap_invl_waiters) > 0)
+ pmap_delayed_invl_finish_unblock(0);
if (invl_gen->saved_pri != 0) {
thread_lock(td);
sched_prio(td, invl_gen->saved_pri);
@@ -888,6 +912,9 @@ DB_SHOW_COMMAND(di_queue, pmap_di_queue)
static long invl_wait;
SYSCTL_LONG(_vm_pmap, OID_AUTO, invl_wait, CTLFLAG_RD, &invl_wait, 0,
"Number of times DI invalidation blocked pmap_remove_all/write");
+static long invl_wait_slow;
+SYSCTL_LONG(_vm_pmap, OID_AUTO, invl_wait_slow, CTLFLAG_RD, &invl_wait_slow, 0,
+ "Number of slow invalidation waits for lockless DI");
#endif
static u_long *
@@ -897,6 +924,27 @@ pmap_delayed_invl_genp(vm_page_t m)
return (&pv_invl_gen[pa_index(VM_PAGE_TO_PHYS(m)) % NPV_LIST_LOCKS]);
}
+static void
+pmap_delayed_invl_callout_func(void *arg __unused)
+{
+
+ if (atomic_load_int(&pmap_invl_waiters) == 0)
+ return;
+ pmap_delayed_invl_finish_unblock(0);
+}
+
+static void
+pmap_delayed_invl_callout_init(void *arg __unused)
+{
+
+ if (pmap_di_locked())
+ return;
+ callout_init(&pmap_invl_callout, 1);
+ pmap_invl_callout_inited = true;
+}
+SYSINIT(pmap_di_callout, SI_SUB_CPU + 1, SI_ORDER_ANY,
+ pmap_delayed_invl_callout_init, NULL);
+
/*
* Ensure that all currently executing DI blocks, that need to flush
* TLB for the given page m, actually flushed the TLB at the time the
@@ -914,7 +962,6 @@ pmap_delayed_invl_genp(vm_page_t m)
static void
pmap_delayed_invl_wait_l(vm_page_t m)
{
- struct turnstile *ts;
u_long *m_gen;
#ifdef PV_STATS
bool accounted = false;
@@ -928,11 +975,7 @@ pmap_delayed_invl_wait_l(vm_page_t m)
accounted = true;
}
#endif
- ts = turnstile_trywait(&invl_gen_ts);
- if (*m_gen > pmap_invl_gen)
- turnstile_wait(ts, NULL, TS_SHARED_QUEUE);
- else
- turnstile_cancel(ts);
+ pmap_delayed_invl_wait_block(m_gen, &pmap_invl_gen);
}
}
@@ -940,19 +983,53 @@ static void
pmap_delayed_invl_wait_u(vm_page_t m)
{
u_long *m_gen;
-#ifdef PV_STATS
- bool accounted = false;
-#endif
+ struct lock_delay_arg lda;
+ bool fast;
+ fast = true;
m_gen = pmap_delayed_invl_genp(m);
+ lock_delay_arg_init(&lda, &di_delay);
while (*m_gen > atomic_load_long(&pmap_invl_gen_head.gen)) {
-#ifdef PV_STATS
- if (!accounted) {
- atomic_add_long(&invl_wait, 1);
- accounted = true;
+ if (fast || !pmap_invl_callout_inited) {
+ PV_STAT(atomic_add_long(&invl_wait, 1));
+ lock_delay(&lda);
+ fast = false;
+ } else {
+ /*
+ * The page's invalidation generation number
+ * is still below the current thread's number.
+ * Prepare to block so that we do not waste
+ * CPU cycles or worse, suffer livelock.
+ *
+ * Since it is impossible to block without
+ * racing with pmap_delayed_invl_finish_u(),
+ * prepare for the race by incrementing
+ * pmap_invl_waiters and arming a 1-tick
+ * callout which will unblock us if we lose
+ * the race.
+ */
+ atomic_add_int(&pmap_invl_waiters, 1);
+
+ /*
+ * Re-check the current thread's invalidation
+ * generation after incrementing
+ * pmap_invl_waiters, so that there is no race
+ * with pmap_delayed_invl_finish_u() setting
+ * the page generation and checking
+ * pmap_invl_waiters. The only race allowed
+ * is for a missed unblock, which is handled
+ * by the callout.
+ */
+ if (*m_gen >
+ atomic_load_long(&pmap_invl_gen_head.gen)) {
+ callout_reset(&pmap_invl_callout, 1,
+ pmap_delayed_invl_callout_func, NULL);
+ PV_STAT(atomic_add_long(&invl_wait_slow, 1));
+ pmap_delayed_invl_wait_block(m_gen,
+ &pmap_invl_gen_head.gen);
+ }
+ atomic_add_int(&pmap_invl_waiters, -1);
}
-#endif
- kern_yield(PRI_USER);
}
}
More information about the svn-src-head
mailing list