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Subject[tip: timers/core] posix-cpu-timers: Get rid of zero checks
The following commit has been merged into the timers/core branch of tip:

Commit-ID: 2bbdbdae05167c688b6d3499a7dab74208b80a22
Gitweb: https://git.kernel.org/tip/2bbdbdae05167c688b6d3499a7dab74208b80a22
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Wed, 21 Aug 2019 21:09:19 +02:00
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Wed, 28 Aug 2019 11:50:40 +02:00

posix-cpu-timers: Get rid of zero checks

Deactivation of the expiry cache is done by setting all clock caches to
0. That requires to have a check for zero in all places which update the
expiry cache:

if (cache == 0 || new < cache)
cache = new;

Use U64_MAX as the deactivated value, which allows to remove the zero
checks when updating the cache and reduces it to the obvious check:

if (new < cache)
cache = new;

This also removes the weird workaround in do_prlimit() which was required
to convert a RLIMIT_CPU value of 0 (immediate expiry) to 1 because handing
in 0 to the posix CPU timer code would have effectively disarmed it.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lkml.kernel.org/r/20190821192922.275086128@linutronix.de

---
include/linux/posix-timers.h | 7 ++++--
kernel/sys.c | 9 +--------
kernel/time/posix-cpu-timers.c | 38 +++++++++++++--------------------
3 files changed, 20 insertions(+), 34 deletions(-)

diff --git a/include/linux/posix-timers.h b/include/linux/posix-timers.h
index 64bd10d..3ea920e 100644
--- a/include/linux/posix-timers.h
+++ b/include/linux/posix-timers.h
@@ -86,7 +86,9 @@ struct posix_cputimers {

static inline void posix_cputimers_init(struct posix_cputimers *pct)
{
- memset(pct->bases, 0, sizeof(pct->bases));
+ pct->bases[0].nextevt = U64_MAX;
+ pct->bases[1].nextevt = U64_MAX;
+ pct->bases[2].nextevt = U64_MAX;
INIT_LIST_HEAD(&pct->bases[0].cpu_timers);
INIT_LIST_HEAD(&pct->bases[1].cpu_timers);
INIT_LIST_HEAD(&pct->bases[2].cpu_timers);
@@ -102,7 +104,8 @@ static inline void posix_cputimers_rt_watchdog(struct posix_cputimers *pct,

/* Init task static initializer */
#define INIT_CPU_TIMERBASE(b) { \
- .cpu_timers = LIST_HEAD_INIT(b.cpu_timers), \
+ .nextevt = U64_MAX, \
+ .cpu_timers = LIST_HEAD_INIT(b.cpu_timers), \
}

#define INIT_CPU_TIMERBASES(b) { \
diff --git a/kernel/sys.c b/kernel/sys.c
index c578b75..2462aa8 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1557,15 +1557,6 @@ int do_prlimit(struct task_struct *tsk, unsigned int resource,
retval = -EPERM;
if (!retval)
retval = security_task_setrlimit(tsk, resource, new_rlim);
- if (resource == RLIMIT_CPU && new_rlim->rlim_cur == 0) {
- /*
- * The caller is asking for an immediate RLIMIT_CPU
- * expiry. But we use the zero value to mean "it was
- * never set". So let's cheat and make it one second
- * instead
- */
- new_rlim->rlim_cur = 1;
- }
}
if (!retval) {
if (old_rlim)
diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
index a738d76..cf85292 100644
--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -122,11 +122,12 @@ static void bump_cpu_timer(struct k_itimer *timer, u64 now)
}
}

-static inline bool expiry_cache_is_zero(const struct posix_cputimers *pct)
+/* Check whether all cache entries contain U64_MAX, i.e. eternal expiry time */
+static inline bool expiry_cache_is_inactive(const struct posix_cputimers *pct)
{
- return !(pct->bases[CPUCLOCK_PROF].nextevt |
- pct->bases[CPUCLOCK_VIRT].nextevt |
- pct->bases[CPUCLOCK_SCHED].nextevt);
+ return !(~pct->bases[CPUCLOCK_PROF].nextevt |
+ ~pct->bases[CPUCLOCK_VIRT].nextevt |
+ ~pct->bases[CPUCLOCK_SCHED].nextevt);
}

static int
@@ -442,11 +443,6 @@ void posix_cpu_timers_exit_group(struct task_struct *tsk)
cleanup_timers(&tsk->signal->posix_cputimers);
}

-static inline int expires_gt(u64 expires, u64 new_exp)
-{
- return expires == 0 || expires > new_exp;
-}
-
/*
* Insert the timer on the appropriate list before any timers that
* expire later. This must be called with the sighand lock held.
@@ -483,7 +479,7 @@ static void arm_timer(struct k_itimer *timer)
* for process timers we share expiration cache with itimers
* and RLIMIT_CPU and for thread timers with RLIMIT_RTTIME.
*/
- if (expires_gt(base->nextevt, newexp))
+ if (newexp < base->nextevt)
base->nextevt = newexp;

if (CPUCLOCK_PERTHREAD(timer->it_clock))
@@ -753,7 +749,7 @@ check_timers_list(struct list_head *timers,
list_move_tail(&t->entry, firing);
}

- return 0;
+ return U64_MAX;
}

static inline void check_dl_overrun(struct task_struct *tsk)
@@ -779,11 +775,7 @@ static void check_thread_timers(struct task_struct *tsk,
if (dl_task(tsk))
check_dl_overrun(tsk);

- /*
- * If the expiry cache is zero, then there are no active per thread
- * CPU timers.
- */
- if (expiry_cache_is_zero(&tsk->posix_cputimers))
+ if (expiry_cache_is_inactive(&tsk->posix_cputimers))
return;

task_cputime(tsk, &utime, &stime);
@@ -833,7 +825,7 @@ static void check_thread_timers(struct task_struct *tsk,
}
}

- if (expiry_cache_is_zero(&tsk->posix_cputimers))
+ if (expiry_cache_is_inactive(&tsk->posix_cputimers))
tick_dep_clear_task(tsk, TICK_DEP_BIT_POSIX_TIMER);
}

@@ -864,7 +856,7 @@ static void check_cpu_itimer(struct task_struct *tsk, struct cpu_itimer *it,
__group_send_sig_info(signo, SEND_SIG_PRIV, tsk);
}

- if (it->expires && (!*expires || it->expires < *expires))
+ if (it->expires && it->expires < *expires)
*expires = it->expires;
}

@@ -948,7 +940,7 @@ static void check_process_timers(struct task_struct *tsk,
}
}
softns = soft * NSEC_PER_SEC;
- if (!prof_exp || softns < prof_exp)
+ if (softns < prof_exp)
prof_exp = softns;
}

@@ -956,7 +948,7 @@ static void check_process_timers(struct task_struct *tsk,
base[CPUCLOCK_VIRT].nextevt = virt_exp;
base[CPUCLOCK_SCHED].nextevt = sched_exp;

- if (expiry_cache_is_zero(&sig->posix_cputimers))
+ if (expiry_cache_is_inactive(&sig->posix_cputimers))
stop_process_timers(sig);

sig->cputimer.checking_timer = false;
@@ -1035,7 +1027,7 @@ task_cputimers_expired(const u64 *sample, struct posix_cputimers *pct)
int i;

for (i = 0; i < CPUCLOCK_MAX; i++) {
- if (pct->bases[i].nextevt && sample[i] >= pct->bases[i].nextevt)
+ if (sample[i] >= pct->bases[i].nextevt)
return true;
}
return false;
@@ -1055,7 +1047,7 @@ static inline bool fastpath_timer_check(struct task_struct *tsk)
{
struct signal_struct *sig;

- if (!expiry_cache_is_zero(&tsk->posix_cputimers)) {
+ if (!expiry_cache_is_inactive(&tsk->posix_cputimers)) {
u64 samples[CPUCLOCK_MAX];

task_sample_cputime(tsk, samples);
@@ -1200,7 +1192,7 @@ void set_process_cpu_timer(struct task_struct *tsk, unsigned int clkid,
* Update expiration cache if this is the earliest timer. CPUCLOCK_PROF
* expiry cache is also used by RLIMIT_CPU!.
*/
- if (expires_gt(*nextevt, *newval))
+ if (*newval < *nextevt)
*nextevt = *newval;

tick_dep_set_signal(tsk->signal, TICK_DEP_BIT_POSIX_TIMER);
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