With preemption turned on we can read incorrect throttling state while being
switched
to CPU on a different chip.
The following BUG_() was hit while running the 4.8-rc5 kernel compiled
with CONFIG_PREEMPT_DEBUG on a POWER machine.
[ 67.700897] BUG: using smp_processor_id() in preemptible [00000000]
code: cat/7343
[ 67.700988] caller is .powernv_cpufreq_throttle_check+0x2c/0x710
[ 67.700998] CPU: 13 PID: 7343 Comm: cat Not tainted 4.8.0-rc5-dirty #1
[ 67.701038] Call Trace:
[ 67.701066] [c0000007d25b75b0] [c000000000971378] .dump_stack+0xe4/0x150
(unreliable)
[ 67.701153] [c0000007d25b7640] [c0000000005162e4]
.check_preemption_disabled+0x134/0x150
[ 67.701238] [c0000007d25b76e0] [c0000000007b63ac]
.powernv_cpufreq_throttle_check+0x2c/0x710
[ 67.701322] [c0000007d25b7790] [c0000000007b6d18]
.powernv_cpufreq_target_index+0x288/0x360
[ 67.701407] [c0000007d25b7870] [c0000000007acee4]
.__cpufreq_driver_target+0x394/0x8c0
[ 67.701491] [c0000007d25b7920] [c0000000007b22ac] .cpufreq_set+0x7c/0xd0
[ 67.701565] [c0000007d25b79b0] [c0000000007adf50]
.store_scaling_setspeed+0x80/0xc0
[ 67.701650] [c0000007d25b7a40] [c0000000007ae270] .store+0xa0/0x100
[ 67.701723] [c0000007d25b7ae0] [c0000000003566e8]
.sysfs_kf_write+0x88/0xb0
[ 67.701796] [c0000007d25b7b70] [c0000000003553b8]
.kernfs_fop_write+0x178/0x260
[ 67.701881] [c0000007d25b7c10] [c0000000002ac3cc] .__vfs_write+0x3c/0x1c0
[ 67.701954] [c0000007d25b7cf0] [c0000000002ad584] .vfs_write+0xc4/0x230
[ 67.702027] [c0000007d25b7d90] [c0000000002aeef8] .SyS_write+0x58/0x100
[ 67.702101] [c0000007d25b7e30] [c00000000000bfec] system_call+0x38/0xfc
The task which updates the cpu-frequency checks if the chip was
throttled. However, it does so without protecting itself against
preemption. Hence, if the task is preempted and woken up on a CPU on
another chip, it can end up updating the throttle statistics of the
incorrect chip.
This patch fixes the issue by ensuring that the throttle_check is
performed with preemption disabled.
Fixes: 09a972d16209 ("cpufreq: powernv: Report cpu frequency throttling")
Reviewed-by: Gautham R. Shenoy <e...@linux.vnet.ibm.com>
Signed-off-by: Denis Kirjanov <k...@linux-powerpc.org>
---
v2: added patch description
drivers/cpufreq/powernv-cpufreq.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/drivers/cpufreq/powernv-cpufreq.c
b/drivers/cpufreq/powernv-cpufreq.c
index d3ffde8..a84724e 100644
--- a/drivers/cpufreq/powernv-cpufreq.c
+++ b/drivers/cpufreq/powernv-cpufreq.c
@@ -647,8 +647,14 @@ static int powernv_cpufreq_target_index(struct
cpufreq_policy *policy,
if (unlikely(rebooting) && new_index != get_nominal_index())
return 0;
- if (!throttled)
+ if (!throttled) {
+ /* we don't want to be preempted while
+ * checking if the CPU frequency has been throttled
+ */
+ preempt_disable();
powernv_cpufreq_throttle_check(NULL);
+ preempt_enable();
+ }
cur_msec = jiffies_to_msecs(get_jiffies_64());
--
1.8.3.1
