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sched, net: Fixup busy_loop_us_clock()

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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@linutronix.de>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: David S. Miller <davem@davemloft.net>
Cc: rui.zhang@intel.com
Cc: jacob.jun.pan@linux.intel.com
Cc: Mike Galbraith <bitbucket@online.de>
Cc: hpa@zytor.com
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: lenb@kernel.org
Cc: rjw@rjwysocki.net
Cc: Eliezer Tamir <eliezer.tamir@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20131119151338.GF3694@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra 2013-11-19 16:13:38 +01:00 • committed by Ingo Molnar
parent 1774e9f3e5
commit 3708983452
1 changed files with 1 additions and 18 deletions
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19
include/net/busy_poll.h
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@ -42,27 +42,10 @@ static inline bool net_busy_loop_on(void)
return sysctl_net_busy_poll; 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) static inline u64 busy_loop_us_clock(void)
{ {
u64 rc; return local_clock() >> 10;
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;
}
#endif /* CONFIG_DEBUG_PREEMPT */
static inline unsigned long sk_busy_loop_end_time(struct sock *sk) static inline unsigned long sk_busy_loop_end_time(struct sock *sk)
{ {