Commit d25f89c9 authored by Jan Kiszka's avatar Jan Kiszka Committed by Anthony Liguori
Browse files

Register Linux dyntick timer as per-thread signal

Derived from kvm-tool patch

Ingo Molnar pointed out that sending the timer signal to the whole
process, just blocking it everywhere, is suboptimal with an increasing
number of threads. QEMU is also using this pattern so far.

Linux provides a (non-portable) way to restrict the signal to a single
thread: We can use SIGEV_THREAD_ID unless we are forced to emulate
signalfd via an additional thread. That case could theoretically be
optimized as well, but it doesn't look worth bothering.
Reviewed-by: default avatarRichard Henderson <>
Signed-off-by: default avatarJan Kiszka <>
Signed-off-by: default avatarAnthony Liguori <>
parent 17604dac
......@@ -115,3 +115,14 @@ int qemu_signalfd(const sigset_t *mask)
return qemu_signalfd_compat(mask);
bool qemu_signalfd_available(void)
errno = 0;
syscall(SYS_signalfd, -1, NULL, _NSIG / 8);
return errno != ENOSYS;
return false;
......@@ -39,5 +39,6 @@ struct qemu_signalfd_siginfo {
int qemu_signalfd(const sigset_t *mask);
bool qemu_signalfd_available(void);
......@@ -831,6 +831,8 @@ static int64_t qemu_next_alarm_deadline(void)
#if defined(__linux__)
#include "compatfd.h"
static int dynticks_start_timer(struct qemu_alarm_timer *t)
struct sigevent ev;
......@@ -850,6 +852,12 @@ static int dynticks_start_timer(struct qemu_alarm_timer *t)
memset(&ev, 0, sizeof(ev));
ev.sigev_value.sival_int = 0;
ev.sigev_notify = SIGEV_SIGNAL;
if (qemu_signalfd_available()) {
ev.sigev_notify = SIGEV_THREAD_ID;
ev._sigev_un._tid = qemu_get_thread_id();
#endif /* SIGEV_THREAD_ID */
ev.sigev_signo = SIGALRM;
if (timer_create(CLOCK_REALTIME, &ev, &host_timer)) {
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