Author: mjg
Date: Sat Jul 25 07:14:33 2020
New Revision: 363511
URL: https://svnweb.freebsd.org/changeset/base/363511
Log:
Do a lockless check in kthread_suspend_check
Otherwise an idle system running lockstat sleep 10 reports contention on
process lock comming from bufdaemon.
While here fix a style nit.
Modified:
head/sys/kern/kern_kthread.c
Modified: head/sys/kern/kern_kthread.c
==============================================================================
--- head/sys/kern/kern_kthread.c Sat Jul 25 06:32:23 2020
(r363510)
+++ head/sys/kern/kern_kthread.c Sat Jul 25 07:14:33 2020
(r363511)
@@ -441,12 +441,15 @@ kthread_suspend_check(void)
panic("%s: curthread is not a valid kthread", __func__);
/*
- * As long as the double-lock protection is used when accessing the
- * TDF_KTH_SUSP flag, synchronizing the read operation via proc mutex
- * is fine.
+ * Setting the TDF_KTH_SUSP flag is protected by process lock.
+ *
+ * Do an unlocked read first to avoid serializing with all other threads
+ * in the common case of not suspending.
*/
+ if ((td->td_flags & TDF_KTH_SUSP) == 0)
+ return;
PROC_LOCK(p);
- while (td->td_flags & TDF_KTH_SUSP) {
+ while ((td->td_flags & TDF_KTH_SUSP) != 0) {
wakeup(&td->td_flags);
msleep(&td->td_flags, &p->p_mtx, PPAUSE, "ktsusp", 0);
}
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