linux-kernel - [tip:sched/idle] sched, net: Fixup busy_loop_us

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Message-ID: <tip-29fc2f66b552770909591d89ee87ab8690d67e61@git.kernel.org>
Date:	Sun, 12 Jan 2014 10:44:39 -0800
From:	tip-bot for Peter Zijlstra <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...nel.org,
	torvalds@...ux-foundation.org, arjan@...ux.intel.com,
	peterz@...radead.org, bitbucket@...ine.de, davem@...emloft.net,
	eliezer.tamir@...ux.intel.com, akpm@...ux-foundation.org,
	tglx@...utronix.de
Subject: [tip:sched/idle] sched, net: Fixup busy_loop_us_clock()

Commit-ID:  29fc2f66b552770909591d89ee87ab8690d67e61
Gitweb:     http://git.kernel.org/tip/29fc2f66b552770909591d89ee87ab8690d67e61
Author:     Peter Zijlstra <peterz@...radead.org>
AuthorDate: Tue, 19 Nov 2013 16:13:38 +0100
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Sun, 12 Jan 2014 14:50:37 +0100

sched, net: Fixup busy_loop_us_clock()

The only valid use of preempt_enable_no_resched() is if the very next
line is schedule() or if we know preemption cannot actually be enabled
by that statement due to known more preempt_count 'refs'.

This busy_poll stuff looks to be completely and utterly broken,
sched_clock() can return utter garbage with interrupts enabled (rare
but still) and it can drift unbounded between CPUs.

This means that if you get preempted/migrated and your new CPU is
years behind on the previous CPU we get to busy spin for a _very_ long
time.

There is a _REASON_ sched_clock() warns about preemptability -
papering over it with a preempt_disable()/preempt_enable_no_resched()
is just terminal brain damage on so many levels.

Replace sched_clock() usage with local_clock() which has a bounded
drift between CPUs (<2 jiffies).

There is a further problem with the entire busy wait poll thing in
that the spin time is additive to the syscall timeout, not inclusive.

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Peter Zijlstra <peterz@...radead.org>
Cc: David S. Miller <davem@...emloft.net>
Cc: rui.zhang@...el.com
Cc: jacob.jun.pan@...ux.intel.com
Cc: Mike Galbraith <bitbucket@...ine.de>
Cc: hpa@...or.com
Cc: Arjan van de Ven <arjan@...ux.intel.com>
Cc: lenb@...nel.org
Cc: rjw@...ysocki.net
Cc: Eliezer Tamir <eliezer.tamir@...ux.intel.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Link: http://lkml.kernel.org/r/20131119151338.GF3694@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 include/net/busy_poll.h | 19 +------------------
 1 file changed, 1 insertion(+), 18 deletions(-)

diff --git a/include/net/busy_poll.h b/include/net/busy_poll.h
index 829627d..1d67fb6 100644
--- a/include/net/busy_poll.h
+++ b/include/net/busy_poll.h
@@ -42,27 +42,10 @@ static inline bool net_busy_loop_on(void)
 	return sysctl_net_busy_poll;
 }
 
-/* a wrapper to make debug_smp_processor_id() happy
- * we can use sched_clock() because we don't care much about precision
- * we only care that the average is bounded
- */
-#ifdef CONFIG_DEBUG_PREEMPT
-static inline u64 busy_loop_us_clock(void)
-{
-	u64 rc;
-
-	preempt_disable_notrace();
-	rc = sched_clock();
-	preempt_enable_no_resched_notrace();
-
-	return rc >> 10;
-}
-#else /* CONFIG_DEBUG_PREEMPT */
 static inline u64 busy_loop_us_clock(void)
 {
-	return sched_clock() >> 10;
+	return local_clock() >> 10;
 }
-#endif /* CONFIG_DEBUG_PREEMPT */
 
 static inline unsigned long sk_busy_loop_end_time(struct sock *sk)
 {
--
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