[PATCH 6/7] sched: Clean up preempt_enable_no_resched() abuse

Peter Zijlstra
2013-11-20T16:33:58Z

[-- Attachment #1: peterz-fixup-weird-preempt_enable_no_resched-usage.patch --]
[-- Type: text/plain, Size: 2815 bytes --]

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'.

As to the busy_poll mess; that looks to be completely and utterly
broken, sched_clock() can return utter garbage with interrupts enabled
(rare but still), it can drift unbounded between CPUs, so 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.

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: Chris Leech <christopher.leech@intel.com>
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: Ingo Molnar <mingo@kernel.org>
Cc: hpa@zytor.com
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
---
include/net/busy_poll.h | 20 ++++++++------------
net/ipv4/tcp.c | 4 ++--
2 files changed, 10 insertions(+), 14 deletions(-)

--- a/include/net/busy_poll.h
+++ b/include/net/busy_poll.h
@@ -42,27 +42,23 @@ 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;

+ /*
+ * XXX with interrupts enabled sched_clock() can return utter garbage
+ * Futhermore, it can have unbounded drift between CPUs, so the below
+ * usage is terminally broken and only serves to shut up a valid debug
+ * warning.
+ */
+
preempt_disable_notrace();
rc = sched_clock();
- preempt_enable_no_resched_notrace();
+ preempt_enable_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)
{
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1623,11 +1623,11 @@ int tcp_recvmsg(struct kiocb *iocb, stru
(len > sysctl_tcp_dma_copybreak) && !(flags & MSG_PEEK) &&
!sysctl_tcp_low_latency &&
net_dma_find_channel()) {
- preempt_enable_no_resched();
+ preempt_enable();
tp->ucopy.pinned_list =
dma_pin_iovec_pages(msg->msg_iov, len);
} else {
- preempt_enable_no_resched();
+ preempt_enable();
}
}
#endif