Commit-ID: 29fc2f66b552770909591d89ee87ab8690d67e61 Gitweb: http://git.kernel.org/tip/29fc2f66b552770909591d89ee87ab8690d67e61 Author: Peter Zijlstra <[email protected]> AuthorDate: Tue, 19 Nov 2013 16:13:38 +0100 Committer: Ingo Molnar <[email protected]> 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 <[email protected]> Signed-off-by: Peter Zijlstra <[email protected]> Cc: David S. Miller <[email protected]> Cc: [email protected] Cc: [email protected] Cc: Mike Galbraith <[email protected]> Cc: [email protected] Cc: Arjan van de Ven <[email protected]> Cc: [email protected] Cc: [email protected] Cc: Eliezer Tamir <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Andrew Morton <[email protected]> Link: http://lkml.kernel.org/r/[email protected] Signed-off-by: Ingo Molnar <[email protected]> --- 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) { -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
