linux-kernel - [patch 39/44] posix-cpu-timers: Get rid of 64bit divisions

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Message-Id: <20190819143805.114825587@linutronix.de>
Date:   Mon, 19 Aug 2019 16:32:20 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        John Stultz <john.stultz@...aro.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>
Subject: [patch 39/44] posix-cpu-timers: Get rid of 64bit divisions

Instead of dividing A to match the units of B it's more efficient to
multiply B to match the units of A.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 kernel/time/posix-cpu-timers.c |   20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -791,10 +791,11 @@ static void check_thread_timers(struct t
 	 */
 	soft = task_rlimit(tsk, RLIMIT_RTTIME);
 	if (soft != RLIM_INFINITY) {
+		/* Task RT timeout is accounted in jiffies. RTTIME is usec */
+		unsigned long rtim = tsk->rt.timeout * (USEC_PER_SEC / HZ);
 		unsigned long hard = task_rlimit_max(tsk, RLIMIT_RTTIME);
 
-		if (hard != RLIM_INFINITY &&
-		    tsk->rt.timeout > DIV_ROUND_UP(hard, USEC_PER_SEC/HZ)) {
+		if (hard != RLIM_INFINITY && rtim >= hard) {
 			/*
 			 * At the hard limit, we just die.
 			 * No need to calculate anything else now.
@@ -806,7 +807,7 @@ static void check_thread_timers(struct t
 			__group_send_sig_info(SIGKILL, SEND_SIG_PRIV, tsk);
 			return;
 		}
-		if (tsk->rt.timeout > DIV_ROUND_UP(soft, USEC_PER_SEC/HZ)) {
+		if (rtim >= soft) {
 			/*
 			 * At the soft limit, send a SIGXCPU every second.
 			 */
@@ -901,11 +902,13 @@ static void check_process_timers(struct
 
 	soft = task_rlimit(tsk, RLIMIT_CPU);
 	if (soft != RLIM_INFINITY) {
+		/* RLIMIT_CPU is in seconds. Samples are nanoseconds */
 		unsigned long hard = task_rlimit_max(tsk, RLIMIT_CPU);
-		u64 softns, ptime = samples[CPUCLOCK_PROF];
-		unsigned long psecs = div_u64(ptime, NSEC_PER_SEC);
+		u64 ptime = samples[CPUCLOCK_PROF];
+		u64 softns = (u64)soft * NSEC_PER_SEC;
+		u64 hardns = (u64)hard * NSEC_PER_SEC;
 
-		if (hard != RLIM_INFINITY && psecs >= hard) {
+		if (hard != RLIM_INFINITY && ptime >= hardns) {
 			/*
 			 * At the hard limit, we just die.
 			 * No need to calculate anything else now.
@@ -917,7 +920,7 @@ static void check_process_timers(struct
 			__group_send_sig_info(SIGKILL, SEND_SIG_PRIV, tsk);
 			return;
 		}
-		if (psecs >= soft) {
+		if (ptime >= softns) {
 			/*
 			 * At the soft limit, send a SIGXCPU every second.
 			 */
@@ -931,7 +934,8 @@ static void check_process_timers(struct
 				sig->rlim[RLIMIT_CPU].rlim_cur = soft;
 			}
 		}
-		softns = soft * NSEC_PER_SEC;
+
+		/* Update the expiry cache */
 		if (softns < exp[CPUCLOCK_PROF])
 			exp[CPUCLOCK_PROF] = softns;
 	}