On 21/07/21(Wed) 09:19, Martin Pieuchot wrote:
> On 23/06/21(Wed) 15:53, Alexander Bluhm wrote:
> > On Wed, Jun 23, 2021 at 11:40:18AM +0200, Martin Pieuchot wrote:
> > > Our previous attempt [0] to replace the current select(2) implementation
> > > has been reverted due to non-acceptable latency increase on sockets [1].
> >
> > I have measured the performance difference.
> >
> > http://bluhm.genua.de/perform/results/2021-06-21T09%3A44%3A18Z/perform.html
> >
> > Worst 20% throughput drop is in 'iperf3 -c10.3.45.35 -u -b10G -w1m
> > -t10 -R' which can be seen here.
> >
> > http://bluhm.genua.de/perform/results/2021-06-21T09%3A44%3A18Z/gnuplot/udp.html
> >
> > Note that iperf3 calls select(2) multiple times per UDP packet.
> >
> > As a new feature I have links to btrace kstack flame graphs in the
> > table.
>
> Thanks a lot for the tests. The FlameGraphs have shown that lazy
> removal wasn't working correctly. Updated diff below now works as
> expected.
>
> I'm aware of the throughput drop in the UDP iperf3 test, this is not a
> real case scenario so I don't consider it as a blocker. However it is
> very useful to check the contention on the NET_LOCK() in select(2). I'm
> working on this issue on another thread, but there's an interdependency
> between the two diffs, due to lock ordering.
Updated diff after recent commits from visa@.
Index: kern/sys_generic.c
===================================================================
RCS file: /cvs/src/sys/kern/sys_generic.c,v
retrieving revision 1.135
diff -u -p -r1.135 sys_generic.c
--- kern/sys_generic.c 8 Jan 2021 09:29:04 -0000 1.135
+++ kern/sys_generic.c 26 Jul 2021 06:56:22 -0000
@@ -55,6 +55,7 @@
#include <sys/time.h>
#include <sys/malloc.h>
#include <sys/poll.h>
+#include <sys/eventvar.h>
#ifdef KTRACE
#include <sys/ktrace.h>
#endif
@@ -66,8 +67,21 @@
#include <uvm/uvm_extern.h>
-int selscan(struct proc *, fd_set *, fd_set *, int, int, register_t *);
-void pollscan(struct proc *, struct pollfd *, u_int, register_t *);
+/*
+ * Debug values:
+ * 1 - print implementation errors, things that should not happen.
+ * 2 - print ppoll(2) information, somewhat verbose
+ * 3 - print pselect(2) and ppoll(2) information, very verbose
+ */
+int kqpoll_debug = 0;
+#define DPRINTFN(v, x...) if (kqpoll_debug > v) { \
+ printf("%s(%d): ", curproc->p_p->ps_comm, curproc->p_tid); \
+ printf(x); \
+}
+
+int pselregister(struct proc *, fd_set *[], int, int *);
+int pselcollect(struct proc *, struct kevent *, fd_set *[], int *);
+
int pollout(struct pollfd *, struct pollfd *, u_int);
int dopselect(struct proc *, int, fd_set *, fd_set *, fd_set *,
struct timespec *, const sigset_t *, register_t *);
@@ -582,13 +596,12 @@ sys_pselect(struct proc *p, void *v, reg
int
dopselect(struct proc *p, int nd, fd_set *in, fd_set *ou, fd_set *ex,
- struct timespec *timeout, const sigset_t *sigmask, register_t *retval)
+ struct timespec *tsp, const sigset_t *sigmask, register_t *retval)
{
+ struct kqueue_scan_state scan;
fd_mask bits[6];
fd_set *pibits[3], *pobits[3];
- struct timespec elapsed, start, stop;
- uint64_t nsecs;
- int s, ncoll, error = 0;
+ int error, n, ncollected = 0, nevents = 0;
u_int ni;
if (nd < 0)
@@ -618,6 +631,8 @@ dopselect(struct proc *p, int nd, fd_set
pobits[2] = (fd_set *)&bits[5];
}
+ kqpoll_init();
+
#define getbits(name, x) \
if (name && (error = copyin(name, pibits[x], ni))) \
goto done;
@@ -636,43 +651,65 @@ dopselect(struct proc *p, int nd, fd_set
if (sigmask)
dosigsuspend(p, *sigmask &~ sigcantmask);
-retry:
- ncoll = nselcoll;
- atomic_setbits_int(&p->p_flag, P_SELECT);
- error = selscan(p, pibits[0], pobits[0], nd, ni, retval);
- if (error || *retval)
+ /* Register kqueue events */
+ error = pselregister(p, pibits, nd, &nevents);
+ if (error != 0)
goto done;
- if (timeout == NULL || timespecisset(timeout)) {
- if (timeout != NULL) {
- getnanouptime(&start);
- nsecs = MIN(TIMESPEC_TO_NSEC(timeout), MAXTSLP);
- } else
- nsecs = INFSLP;
- s = splhigh();
- if ((p->p_flag & P_SELECT) == 0 || nselcoll != ncoll) {
- splx(s);
- goto retry;
- }
- atomic_clearbits_int(&p->p_flag, P_SELECT);
- error = tsleep_nsec(&selwait, PSOCK | PCATCH, "select", nsecs);
- splx(s);
- if (timeout != NULL) {
- getnanouptime(&stop);
- timespecsub(&stop, &start, &elapsed);
- timespecsub(timeout, &elapsed, timeout);
- if (timeout->tv_sec < 0)
- timespecclear(timeout);
+
+ /*
+ * The poll/select family of syscalls has been designed to
+ * block when file descriptors are not available, even if
+ * there's nothing to wait for.
+ */
+ if (nevents == 0) {
+ uint64_t nsecs = INFSLP;
+
+ if (tsp != NULL) {
+ if (!timespecisset(tsp))
+ goto done;
+ nsecs = MAX(1, MIN(TIMESPEC_TO_NSEC(tsp), MAXTSLP));
}
- if (error == 0 || error == EWOULDBLOCK)
- goto retry;
+ error = tsleep_nsec(&p->p_kq, PSOCK | PCATCH, "kqsel", nsecs);
+ /* select is not restarted after signals... */
+ if (error == ERESTART)
+ error = EINTR;
+ if (error == EWOULDBLOCK)
+ error = 0;
+ goto done;
+ }
+
+ /* Collect at most `nevents' possibly waiting in kqueue_scan() */
+ kqueue_scan_setup(&scan, p->p_kq);
+ while ((n = nevents - ncollected) > 0) {
+ struct kevent kev[KQ_NEVENTS];
+ int i, ready;
+
+ if (n > nitems(kev))
+ n = nitems(kev);
+ ready = kqueue_scan(&scan, nitems(kev), kev, tsp, p, &error);
+ if (ready == 0)
+ break;
+#ifdef KTRACE
+ if (KTRPOINT(p, KTR_STRUCT))
+ ktrevent(p, kev, ready);
+#endif
+ /* Convert back events that are ready/delete spurious ones. */
+ for (i = 0; i < ready && error == 0; i++)
+ error = pselcollect(p, &kev[i], pobits, &ncollected);
+
+ if (error)
+ break;
+
+ /*
+ * If we only got spurious events, try again to wait at
+ * least once.
+ */
+ if (ncollected == 0 && ready > 0)
+ scan.kqs_nevent = 0;
}
-done:
- atomic_clearbits_int(&p->p_flag, P_SELECT);
- /* select is not restarted after signals... */
- if (error == ERESTART)
- error = EINTR;
- if (error == EWOULDBLOCK)
- error = 0;
+ kqueue_scan_finish(&scan);
+ *retval = ncollected;
+ done:
#define putbits(name, x) \
if (name && (error2 = copyout(pobits[x], name, ni))) \
error = error2;
@@ -694,40 +731,108 @@ done:
if (pibits[0] != (fd_set *)&bits[0])
free(pibits[0], M_TEMP, 6 * ni);
+
+ /* Needed to remove events lazily. */
+ p->p_kq_serial += nd;
+
return (error);
}
+/*
+ * Convert fd_set into kqueue events and register them on the
+ * per-thread queue.
+ */
int
-selscan(struct proc *p, fd_set *ibits, fd_set *obits, int nfd, int ni,
- register_t *retval)
+pselregister(struct proc *p, fd_set *pibits[3], int nfd, int *nregistered)
{
- caddr_t cibits = (caddr_t)ibits, cobits = (caddr_t)obits;
- struct filedesc *fdp = p->p_fd;
- int msk, i, j, fd;
+ static const int evf[] = { EVFILT_READ, EVFILT_WRITE, EVFILT_EXCEPT };
+ static const int evff[] = { 0, 0, NOTE_OOB };
+ int msk, i, j, fd, nevents = 0, error = 0;
+ struct kevent kev;
fd_mask bits;
- struct file *fp;
- int n = 0;
- static const int flag[3] = { POLLIN, POLLOUT|POLL_NOHUP, POLLPRI };
for (msk = 0; msk < 3; msk++) {
- fd_set *pibits = (fd_set *)&cibits[msk*ni];
- fd_set *pobits = (fd_set *)&cobits[msk*ni];
-
for (i = 0; i < nfd; i += NFDBITS) {
- bits = pibits->fds_bits[i/NFDBITS];
+ bits = pibits[msk]->fds_bits[i / NFDBITS];
while ((j = ffs(bits)) && (fd = i + --j) < nfd) {
bits &= ~(1 << j);
- if ((fp = fd_getfile(fdp, fd)) == NULL)
- return (EBADF);
- if ((*fp->f_ops->fo_poll)(fp, flag[msk], p)) {
- FD_SET(fd, pobits);
- n++;
+
+ DPRINTFN(2, "select fd %d mask %d serial %lu\n",
+ fd, msk, p->p_kq_serial);
+ EV_SET(&kev, fd, evf[msk],
+ EV_ADD|EV_ENABLE|__EV_POLL,
+ evff[msk], 0, (void *)(p->p_kq_serial));
+#ifdef KTRACE
+ if (KTRPOINT(p, KTR_STRUCT))
+ ktrevent(p, &kev, 1);
+#endif
+ error = kqueue_register(p->p_kq, &kev, p);
+ switch (error) {
+ case 0:
+ nevents++;
+ /* FALLTHROUGH */
+ case EOPNOTSUPP:/* No underlying kqfilter */
+ case EINVAL: /* Unimplemented filter */
+ error = 0;
+ break;
+ case ENXIO: /* Device has been detached */
+ default:
+ goto bad;
}
- FRELE(fp, p);
}
}
}
- *retval = n;
+
+ *nregistered = nevents;
+ return (0);
+bad:
+ DPRINTFN(0, "select fd %u filt %d error %d\n", (int)kev.ident,
+ kev.filter, error);
+ return (error);
+}
+
+/*
+ * Convert given kqueue event into corresponding select(2) bit.
+ */
+int
+pselcollect(struct proc *p, struct kevent *kevp, fd_set *pobits[3],
+ int *ncollected)
+{
+#ifdef DIAGNOSTIC
+ /* Filter out and lazily delete spurious events */
+ if ((unsigned long)kevp->udata != p->p_kq_serial) {
+ DPRINTFN(0, "select fd %u lazily deleted (%lu != %lu)\n",
+ (int)kevp->ident, p->p_kq_serial,
+ (unsigned long)kevp->udata);
+ kevp->flags = EV_DISABLE|EV_DELETE;
+ kqueue_register(p->p_kq, kevp, p);
+ return (0);
+ }
+#endif
+ if (kevp->flags & EV_ERROR) {
+ DPRINTFN(2, "select fd %d filt %d error %d\n",
+ (int)kevp->ident, kevp->filter, (int)kevp->data);
+ kevp->flags = EV_DISABLE|EV_DELETE;
+ kqueue_register(p->p_kq, kevp, p);
+ return (kevp->data);
+ }
+
+ switch (kevp->filter) {
+ case EVFILT_READ:
+ FD_SET(kevp->ident, pobits[0]);
+ break;
+ case EVFILT_WRITE:
+ FD_SET(kevp->ident, pobits[1]);
+ break;
+ case EVFILT_EXCEPT:
+ FD_SET(kevp->ident, pobits[2]);
+ break;
+ default:
+ KASSERT(0);
+ }
+ (*ncollected)++;
+
+ DPRINTFN(2, "select fd %d filt %d\n", (int)kevp->ident, kevp->filter);
return (0);
}
