NetBSD Problem Report #57512
From conorh@sdf.org Fri Jul 7 21:43:02 2023
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Message-Id: <202307072142.367Lgo0w023311@sdf.org>
Date: Fri, 7 Jul 2023 14:42:50 -0700 (PDT)
From: conorh@sdf.org
Reply-To: conorh@sdf.org
To: gnats-bugs@NetBSD.org
Subject: clock_gettime(CLOCK_THREAD_CPUTIME_ID) sometimes goes backwards
X-Send-Pr-Version: 3.95
>Number: 57512
>Category: kern
>Synopsis: clock_gettime(CLOCK_THREAD_CPUTIME_ID, &ts) sometimes reports a timespec less than one obtained on a previous invocation (i.e, it goes backwards)
>Confidential: no
>Severity: non-critical
>Priority: low
>Responsible: kern-bug-people
>State: open
>Class: sw-bug
>Submitter-Id: net
>Arrival-Date: Fri Jul 07 21:45:00 +0000 2023
>Last-Modified: Sat Jul 08 20:05:01 +0000 2023
>Originator: Conor Hughes
>Release: NetBSD 9.3
>Organization:
conorh@sdf.org
SDF Public Access UNIX System - http://sdf.org
>Environment:
System: NetBSD otaku 9.3 NetBSD 9.3 (GENERIC) #0: Thu Aug 4 15:30:37 UTC 2022 mkrepro@mkrepro.NetBSD.org:/usr/src/sys/arch/amd64/compile/GENERIC amd64
Architecture: x86_64
Machine: amd64
>Description:
When calling clock_gettime(2) and using CLOCK_THREAD_CPUTIME_ID as the clockid_t, the value filled into the timespec is sometimes less than a previous value obtained via the same call on the same thread. Since consuming negative CPU time doesn't make sense, this seems like a bug.
CLOCK_PROCESS_CPUTIME_ID does not seem to be affected and is a workaround in some cases. I've reproduced this on NetBSD 9.3 on x86_64 and evbarm/armv6.
>How-To-Repeat:
Compile and run the following test program.
#include <stdio.h>
#include <time.h>
#include <err.h>
#include <sysexits.h>
#include <stdlib.h>
#include <string.h>
/* The specifics of this function are immaterial but we have to do some work in order to separate
* start and end.
*/
static void waste_some_time()
{
for ( int i = 0; i< 1000; i++ ) {
char buf[4 * 4096];
arc4random_buf(buf, sizeof(buf));
char buf2[4 * 4096];
arc4random_buf(buf2, sizeof(buf2));
if ( memcmp(buf, buf2, sizeof(buf)) == 0 ) {
fprintf(stderr, "wow!\n");
}
}
}
int main(void)
{
static const int niter = 100;
static const clockid_t query_clock = CLOCK_THREAD_CPUTIME_ID;
for ( int iter = 0; iter < niter; iter++ ) {
struct timespec start;
if ( clock_gettime(query_clock, &start) != 0 ) {
err(EX_OSERR, "clock_gettime");
}
waste_some_time();
struct timespec end;
if ( clock_gettime(query_clock, &end) != 0 ) {
err(EX_OSERR, "clock_gettime");
}
if ( end.tv_sec < start.tv_sec ||
(end.tv_sec == start.tv_sec && end.tv_nsec < start.tv_nsec) ) {
errx(EX_OSERR, "clock_gettime went backwards!? after %d iterations, start (%lld %lld) end (%lld %lld)",
iter,
(long long)start.tv_sec, (long long)start.tv_nsec,
(long long)end.tv_sec, (long long)end.tv_nsec);
}
}
fprintf(stderr, "did %d iterations.\n", niter);
return EXIT_SUCCESS;
}
>Fix:
Unknown
>Audit-Trail:
From: Taylor R Campbell <riastradh@NetBSD.org>
To: conorh@sdf.org
Cc: gnats-bugs@NetBSD.org
Subject: Re: kern/57512: clock_gettime(CLOCK_THREAD_CPUTIME_ID) sometimes goes backwards
Date: Sat, 8 Jul 2023 00:18:21 +0000
This is because CLOCK_THREAD_CPUTIME_ID is implemented by summing
l->l_rticksum + l->l_slpticksum, but these are both reset periodically
by sched_lwp_stats.
Aside: Not sure why we add the sleeping time, l_slpticksum?
I think this should probably instead use l->l_ru.ru_utime +
l->l_ru.ru_stime, possibly with an adjustment from the timecounter
since the last tick like calcru does for process getrusage. But I
would need to think more about it to be sure.
From: Taylor R Campbell <riastradh@NetBSD.org>
To: conorh@sdf.org
Cc: gnats-bugs@NetBSD.org
Subject: Re: kern/57512: clock_gettime(CLOCK_THREAD_CPUTIME_ID) sometimes goes backwards
Date: Sat, 8 Jul 2023 01:31:03 +0000
This is a multi-part message in MIME format.
--=_17Wymd7lPaKOmlAiVMiz9N/EY5eA0fm1
Can you please try the attached patch?
--=_17Wymd7lPaKOmlAiVMiz9N/EY5eA0fm1
Content-Type: text/plain; charset="ISO-8859-1"; name="clockcputimeid"
Content-Transfer-Encoding: quoted-printable
Content-Disposition: attachment; filename="clockcputimeid.patch"
From 881ce53d36439f2dc9a4cc3be389095344d42501 Mon Sep 17 00:00:00 2001
From: Taylor R Campbell <riastradh@NetBSD.org>
Date: Sat, 8 Jul 2023 01:29:28 +0000
Subject: [PATCH] WIP: clock_gettime(2): Fix CLOCK_PROCESS/THREAD_CPUTIME_ID.
Use same calculation as getrusage, not some ad-hoc arithmetic of
internal scheduler parameters that are periodically rewound.
XXX This creates copypasta of calcru for lwps -- should factor it out
into a calcrulwp or something to avoid this.
---
sys/kern/subr_time.c | 67 ++++++++++++++++++++++----------------------
1 file changed, 33 insertions(+), 34 deletions(-)
diff --git a/sys/kern/subr_time.c b/sys/kern/subr_time.c
index eaa8312dd8fb..cad7fa26e998 100644
--- a/sys/kern/subr_time.c
+++ b/sys/kern/subr_time.c
@@ -45,12 +45,6 @@ __KERNEL_RCSID(0, "$NetBSD: subr_time.c,v 1.37 2023/04/2=
9 03:36:55 isaki Exp $")
#include <sys/timetc.h>
#include <sys/intr.h>
=20
-#ifdef DEBUG_STICKS
-#define DPRINTF(a) uprintf a
-#else
-#define DPRINTF(a)=20
-#endif
-
/*
* Compute number of hz until specified time. Used to compute second
* argument to callout_reset() from an absolute time.
@@ -248,32 +242,16 @@ clock_timeleft(clockid_t clockid, struct timespec *ts=
, struct timespec *sleepts)
*sleepts =3D sleptts;
}
=20
-static void
-ticks2ts(uint64_t ticks, struct timespec *ts)
-{
- ts->tv_sec =3D ticks / hz;
- uint64_t sticks =3D ticks - ts->tv_sec * hz;
- if (sticks > BINTIME_SCALE_MS) /* floor(2^64 / 1000) */
- ts->tv_nsec =3D sticks / hz * 1000000000LL;
- else if (sticks > BINTIME_SCALE_US) /* floor(2^64 / 1000000) */
- ts->tv_nsec =3D sticks * 1000LL / hz * 1000000LL;
- else
- ts->tv_nsec =3D sticks * 1000000000LL / hz;
- DPRINTF(("%s: %ju/%ju -> %ju.%ju\n", __func__,
- (uintmax_t)ticks, (uintmax_t)sticks,
- (uintmax_t)ts->tv_sec, (uintmax_t)ts->tv_nsec));
-}
-
int
clock_gettime1(clockid_t clock_id, struct timespec *ts)
{
int error;
- uint64_t ticks;
struct proc *p;
=20
#define CPUCLOCK_ID_MASK (~(CLOCK_THREAD_CPUTIME_ID|CLOCK_PROCESS_CPUTIME_=
ID))
if (clock_id & CLOCK_PROCESS_CPUTIME_ID) {
pid_t pid =3D clock_id & CPUCLOCK_ID_MASK;
+ struct timeval cputime;
=20
mutex_enter(&proc_lock);
p =3D pid =3D=3D 0 ? curproc : proc_find(pid);
@@ -281,10 +259,10 @@ clock_gettime1(clockid_t clock_id, struct timespec *t=
s)
mutex_exit(&proc_lock);
return ESRCH;
}
- ticks =3D p->p_uticks + p->p_sticks + p->p_iticks;
- DPRINTF(("%s: u=3D%ju, s=3D%ju, i=3D%ju\n", __func__,
- (uintmax_t)p->p_uticks, (uintmax_t)p->p_sticks,
- (uintmax_t)p->p_iticks));
+ mutex_enter(p->p_lock);
+ calcru(p, /*usertime*/NULL, /*systime*/NULL, /*intrtime*/NULL,
+ &cputime);
+ mutex_exit(p->p_lock);
mutex_exit(&proc_lock);
=20
// XXX: Perhaps create a special kauth type
@@ -293,9 +271,14 @@ clock_gettime1(clockid_t clock_id, struct timespec *ts)
KAUTH_ARG(KAUTH_REQ_PROCESS_CANSEE_ENTRY), NULL, NULL);
if (error)
return error;
+
+ TIMEVAL_TO_TIMESPEC(&cputime, ts);
+ return 0;
} else if (clock_id & CLOCK_THREAD_CPUTIME_ID) {
struct lwp *l;
lwpid_t lid =3D clock_id & CPUCLOCK_ID_MASK;
+ struct bintime tm =3D {0, 0};
+
p =3D curproc;
mutex_enter(p->p_lock);
l =3D lid =3D=3D 0 ? curlwp : lwp_find(p, lid);
@@ -303,15 +286,31 @@ clock_gettime1(clockid_t clock_id, struct timespec *t=
s)
mutex_exit(p->p_lock);
return ESRCH;
}
- ticks =3D l->l_rticksum + l->l_slpticksum;
- DPRINTF(("%s: r=3D%ju, s=3D%ju\n", __func__,
- (uintmax_t)l->l_rticksum, (uintmax_t)l->l_slpticksum));
+
+ /* XXX begin copypasta from calcru */
+ lwp_lock(l);
+ bintime_add(&tm, &l->l_rtime);
+ if ((l->l_pflag & LP_RUNNING) !=3D 0 &&
+ (l->l_pflag & (LP_INTR | LP_TIMEINTR)) !=3D LP_INTR) {
+ struct bintime diff;
+ /*
+ * Adjust for the current time slice. This is
+ * actually fairly important since the error
+ * here is on the order of a time quantum,
+ * which is much greater than the sampling
+ * error.
+ */
+ binuptime(&diff);
+ membar_consumer(); /* for softint_dispatch() */
+ bintime_sub(&diff, &l->l_stime);
+ bintime_add(&tm, &diff);
+ }
+ lwp_unlock(l);
+ /* XXX end copypasta from calcru */
+
mutex_exit(p->p_lock);
- } else
- ticks =3D (uint64_t)-1;
=20
- if (ticks !=3D (uint64_t)-1) {
- ticks2ts(ticks, ts);
+ bintime2timespec(&tm, ts);
return 0;
}
=20
--=_17Wymd7lPaKOmlAiVMiz9N/EY5eA0fm1--
From: "Taylor R Campbell" <riastradh@netbsd.org>
To: gnats-bugs@gnats.NetBSD.org
Cc:
Subject: PR/57512 CVS commit: src/tests/lib/libc/sys
Date: Sat, 8 Jul 2023 14:05:51 +0000
Module Name: src
Committed By: riastradh
Date: Sat Jul 8 14:05:51 UTC 2023
Modified Files:
src/tests/lib/libc/sys: t_clock_gettime.c
Log Message:
t_clock_gettime: Add test for PR kern/57512.
To generate a diff of this commit:
cvs rdiff -u -r1.3 -r1.4 src/tests/lib/libc/sys/t_clock_gettime.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
From: "Taylor R Campbell" <riastradh@netbsd.org>
To: gnats-bugs@gnats.NetBSD.org
Cc:
Subject: PR/57512 CVS commit: src
Date: Sat, 8 Jul 2023 20:02:10 +0000
Module Name: src
Committed By: riastradh
Date: Sat Jul 8 20:02:10 UTC 2023
Modified Files:
src/sys/kern: kern_resource.c subr_time.c
src/sys/sys: resourcevar.h
src/tests/lib/libc/sys: t_clock_gettime.c
Log Message:
clock_gettime(2): Fix CLOCK_PROCESS/THREAD_CPUTIME_ID.
Use same calculation as getrusage, not some ad-hoc arithmetic of
internal scheduler parameters that are periodically rewound.
PR kern/57512
XXX pullup-8
XXX pullup-9
XXX pullup-10
To generate a diff of this commit:
cvs rdiff -u -r1.190 -r1.191 src/sys/kern/kern_resource.c
cvs rdiff -u -r1.37 -r1.38 src/sys/kern/subr_time.c
cvs rdiff -u -r1.57 -r1.58 src/sys/sys/resourcevar.h
cvs rdiff -u -r1.4 -r1.5 src/tests/lib/libc/sys/t_clock_gettime.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
(Contact us)
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