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From: John Stultz <john.stultz@linaro.org>
To: lkml <linux-kernel@vger.kernel.org>
Cc: John Stultz <john.stultz@linaro.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	stable <stable@vger.kernel.org>,
	Brendan Gregg <bgregg@netflix.com>,
	Alexei Starovoitov <alexei.starovoitov@gmail.com>
Subject: [PATCH] timekeeping: Fix __ktime_get_fast_ns() regression
Date: Tue,  4 Oct 2016 19:55:48 -0700
Message-ID: <1475636148-26539-1-git-send-email-john.stultz@linaro.org> (raw)
In-Reply-To: <20161005003049.GA75907@ast-mbp.thefacebook.com>

In commit 27727df240c7 ("Avoid taking lock in NMI path with
CONFIG_DEBUG_TIMEKEEPING"), I changed the logic to open-code
the timekeeping_get_ns() function, but I forgot to include
the unit conversion from cycles to nanoseconds, breaking the
function's output, which impacts users like perf.

This would result in bogus perf timestamps like:
 swapper     0 [000]   253.427536:  111111111 cpu-clock:  ffffffff810a0de6 native_safe_halt+0x6 ([kernel.kallsyms])
 swapper     0 [000]   254.426573:  111111111 cpu-clock:  ffffffff810a0de6 native_safe_halt+0x6 ([kernel.kallsyms])
 swapper     0 [000]   254.426687:  111111111 cpu-clock:  ffffffff810a0de6 native_safe_halt+0x6 ([kernel.kallsyms])
 swapper     0 [000]   254.426800:  111111111 cpu-clock:  ffffffff810a0de6 native_safe_halt+0x6 ([kernel.kallsyms])
 swapper     0 [000]   254.426905:  111111111 cpu-clock:  ffffffff810a0de6 native_safe_halt+0x6 ([kernel.kallsyms])
 swapper     0 [000]   254.427022:  111111111 cpu-clock:  ffffffff810a0de6 native_safe_halt+0x6 ([kernel.kallsyms])
 swapper     0 [000]   254.427127:  111111111 cpu-clock:  ffffffff810a0de6 native_safe_halt+0x6 ([kernel.kallsyms])
 swapper     0 [000]   254.427239:  111111111 cpu-clock:  ffffffff810a0de6 native_safe_halt+0x6 ([kernel.kallsyms])
 swapper     0 [000]   254.427346:  111111111 cpu-clock:  ffffffff810a0de6 native_safe_halt+0x6 ([kernel.kallsyms])
 swapper     0 [000]   254.427463:  111111111 cpu-clock:  ffffffff810a0de6 native_safe_halt+0x6 ([kernel.kallsyms])
 swapper     0 [000]   255.426572:  111111111 cpu-clock:  ffffffff810a0de6 native_safe_halt+0x6 ([kernel.kallsyms])

Instead of more reasonable expected timestamps like:
 swapper     0 [000]    39.953768:  111111111 cpu-clock:  ffffffff810a0de6 native_safe_halt+0x6 ([kernel.kallsyms])
 swapper     0 [000]    40.064839:  111111111 cpu-clock:  ffffffff810a0de6 native_safe_halt+0x6 ([kernel.kallsyms])
 swapper     0 [000]    40.175956:  111111111 cpu-clock:  ffffffff810a0de6 native_safe_halt+0x6 ([kernel.kallsyms])
 swapper     0 [000]    40.287103:  111111111 cpu-clock:  ffffffff810a0de6 native_safe_halt+0x6 ([kernel.kallsyms])
 swapper     0 [000]    40.398217:  111111111 cpu-clock:  ffffffff810a0de6 native_safe_halt+0x6 ([kernel.kallsyms])
 swapper     0 [000]    40.509324:  111111111 cpu-clock:  ffffffff810a0de6 native_safe_halt+0x6 ([kernel.kallsyms])
 swapper     0 [000]    40.620437:  111111111 cpu-clock:  ffffffff810a0de6 native_safe_halt+0x6 ([kernel.kallsyms])
 swapper     0 [000]    40.731546:  111111111 cpu-clock:  ffffffff810a0de6 native_safe_halt+0x6 ([kernel.kallsyms])
 swapper     0 [000]    40.842654:  111111111 cpu-clock:  ffffffff810a0de6 native_safe_halt+0x6 ([kernel.kallsyms])
 swapper     0 [000]    40.953772:  111111111 cpu-clock:  ffffffff810a0de6 native_safe_halt+0x6 ([kernel.kallsyms])
 swapper     0 [000]    41.064881:  111111111 cpu-clock:  ffffffff810a0de6 native_safe_halt+0x6 ([kernel.kallsyms])

This patch adds the proper use of timekeeping_delta_to_ns()
to convert the cycle delta to nanoseconds as needed.

Thanks to Brendan and Alexei for finding this quickly after
the v4.8 release. Unfortunately the problematic commit has
landed in some -stable trees so they'll need this fix as
well.

Many apologies for this mistake. I'll be looking to add a
perf-clock sanity test to the kselftest timers tests soon.

Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable <stable@vger.kernel.org>
Cc: Brendan Gregg <bgregg@netflix.com>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Fixes: 27727df240c7 "timekeeping: Avoid taking lock in NMI path with CONFIG_DEBUG_TIMEKEEPING"
Reported-by: Brendan Gregg <bgregg@netflix.com>
Reported-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
---
Thomas/Ingo:
I've reproduced the issue and validated this fixes it, but given my limited perf
usage so far, I'd appreciate waiting for a Tested-by: from one of the reporters
before considering for tip/timers/urgent.

 kernel/time/timekeeping.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index e07fb09..37dec7e 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -403,8 +403,11 @@ static __always_inline u64 __ktime_get_fast_ns(struct tk_fast *tkf)
 		tkr = tkf->base + (seq & 0x01);
 		now = ktime_to_ns(tkr->base);
 
-		now += clocksource_delta(tkr->read(tkr->clock),
-					 tkr->cycle_last, tkr->mask);
+		now += timekeeping_delta_to_ns(tkr,
+				clocksource_delta(
+					tkr->read(tkr->clock),
+					tkr->cycle_last,
+					tkr->mask));
 	} while (read_seqcount_retry(&tkf->seq, seq));
 
 	return now;
-- 
1.9.1

  parent reply index

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-08-23 23:08 [GIT PULL][PATCH 0/2] Timekeeping items for 4.8 via tip/timers/urgent John Stultz
2016-08-23 23:08 ` [PATCH 1/2] timekeeping: Avoid taking lock in NMI path with CONFIG_DEBUG_TIMEKEEPING John Stultz
2016-08-24  7:40   ` [tip:timers/urgent] " tip-bot for John Stultz
2016-10-05  0:30   ` [PATCH 1/2] " Alexei Starovoitov
2016-10-05  0:50     ` John Stultz
2016-10-05  2:55     ` John Stultz [this message]
2016-10-05 10:57       ` [PATCH] timekeeping: Fix __ktime_get_fast_ns() regression Mathieu Desnoyers
2016-10-05 13:48       ` [tip:timers/urgent] " tip-bot for John Stultz
2016-08-23 23:08 ` [PATCH 2/2] timekeeping: Cap array access in timekeeping_debug to protect against invalid sleep times John Stultz
2016-08-24  0:58   ` Chen Yu
2016-08-24  2:54     ` John Stultz
2016-08-24  2:56   ` [RFC][PATCH v2] " John Stultz
2016-08-24  7:41   ` [tip:timers/urgent] timekeeping: Cap array access in timekeeping_debug tip-bot for John Stultz

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