[lkml]   [2013]   [Nov]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
Patch in this message
Subject[PATCH 7/8] 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

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.

Cc: David S. Miller <>
Cc: Mike Galbraith <>
Cc: Ingo Molnar <>
Cc: Arjan van de Ven <>
Cc: Eliezer Tamir <>
Reviewed-by: Thomas Gleixner <>
Signed-off-by: Peter Zijlstra <>
include/net/busy_poll.h | 19 +------------------
1 file changed, 1 insertion(+), 18 deletions(-)

--- 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
- */
static inline u64 busy_loop_us_clock(void)
- u64 rc;
- preempt_disable_notrace();
- rc = sched_clock();
- preempt_enable_no_resched_notrace();
- return rc >> 10;
-static inline u64 busy_loop_us_clock(void)
- return sched_clock() >> 10;
+ return local_clock() >> 10;

static inline unsigned long sk_busy_loop_end_time(struct sock *sk)

 \ /
  Last update: 2013-11-26 17:41    [W:0.090 / U:2.600 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site