linux-kernel - [patch 06/14] tasklets: Replace spin wait in tasklet

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Date:   Tue, 09 Mar 2021 09:42:09 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Denis Kirjanov <kda@...ux-powerpc.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        ath9k-devel@....qualcomm.com, Kalle Valo <kvalo@...eaurora.org>,
        linux-wireless@...r.kernel.org, Chas Williams <3chas3@...il.com>,
        linux-atm-general@...ts.sourceforge.net,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-hyperv@...r.kernel.org, linux-pci@...r.kernel.org,
        Stefan Richter <stefanr@...6.in-berlin.de>,
        linux1394-devel@...ts.sourceforge.net
Subject: [patch 06/14] tasklets: Replace spin wait in tasklet_kill()

From: Peter Zijlstra <peterz@...radead.org>

tasklet_kill() spin waits for TASKLET_STATE_SCHED to be cleared invoking
yield() from inside the loop. yield() is an ill defined mechanism and the
result might still be wasting CPU cycles in a tight loop which is
especially painful in a guest when the CPU running the tasklet is scheduled
out.

tasklet_kill() is used in teardown paths and not performance critical at
all. Replace the spin wait with wait_var_event().

Signed-off-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 kernel/softirq.c |   23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -532,6 +532,16 @@ void __tasklet_hi_schedule(struct taskle
 }
 EXPORT_SYMBOL(__tasklet_hi_schedule);
 
+static inline bool tasklet_clear_sched(struct tasklet_struct *t)
+{
+	if (test_and_clear_bit(TASKLET_STATE_SCHED, &t->state)) {
+		wake_up_var(&t->state);
+		return true;
+	}
+
+	return false;
+}
+
 static void tasklet_action_common(struct softirq_action *a,
 				  struct tasklet_head *tl_head,
 				  unsigned int softirq_nr)
@@ -551,8 +561,7 @@ static void tasklet_action_common(struct
 
 		if (tasklet_trylock(t)) {
 			if (!atomic_read(&t->count)) {
-				if (!test_and_clear_bit(TASKLET_STATE_SCHED,
-							&t->state))
+				if (!tasklet_clear_sched(t))
 					BUG();
 				if (t->use_callback)
 					t->callback(t);
@@ -612,13 +621,11 @@ void tasklet_kill(struct tasklet_struct
 	if (in_interrupt())
 		pr_notice("Attempt to kill tasklet from interrupt\n");
 
-	while (test_and_set_bit(TASKLET_STATE_SCHED, &t->state)) {
-		do {
-			yield();
-		} while (test_bit(TASKLET_STATE_SCHED, &t->state));
-	}
+	while (test_and_set_bit(TASKLET_STATE_SCHED, &t->state))
+		wait_var_event(&t->state, !test_bit(TASKLET_STATE_SCHED, &t->state));
+
 	tasklet_unlock_wait(t);
-	clear_bit(TASKLET_STATE_SCHED, &t->state);
+	tasklet_clear_sched(t);
 }
 EXPORT_SYMBOL(tasklet_kill);