Commit-ID: 37089834528be3ef8cbf927e47c753b3e272a856 Gitweb: http://git.kernel.org/tip/37089834528be3ef8cbf927e47c753b3e272a856 Author: Peter Zijlstra <pet...@infradead.org> AuthorDate: Tue, 19 Nov 2013 16:13:38 +0100 Committer: Ingo Molnar <mi...@kernel.org> CommitDate: Mon, 13 Jan 2014 17:39:11 +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 <t...@linutronix.de> Signed-off-by: Peter Zijlstra <pet...@infradead.org> Cc: David S. Miller <da...@davemloft.net> Cc: rui.zh...@intel.com Cc: jacob.jun....@linux.intel.com Cc: Mike Galbraith <bitbuc...@online.de> Cc: h...@zytor.com Cc: Arjan van de Ven <ar...@linux.intel.com> Cc: l...@kernel.org Cc: r...@rjwysocki.net Cc: Eliezer Tamir <eliezer.ta...@linux.intel.com> Cc: Linus Torvalds <torva...@linux-foundation.org> Cc: Andrew Morton <a...@linux-foundation.org> Link: http://lkml.kernel.org/r/20131119151338.gf3...@twins.programming.kicks-ass.net Signed-off-by: Ingo Molnar <mi...@kernel.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) { -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
