[PATCHv2 2/4] kernel/watchdog_hld: clarify the condition in hardlockup_detector_event_create()
Petr Mladek
pmladek at suse.com
Mon Oct 4 05:32:47 PDT 2021
On Thu 2021-09-23 22:09:49, Pingfan Liu wrote:
> As for the context, there are two arguments to change
> debug_smp_processor_id() to is_percpu_thread().
>
> -1. watchdog_ev is percpu, and migration will frustrate the attempt
> which try to bind a watchdog_ev to a cpu by protecting this func inside
> the pair of preempt_disable()/preempt_enable().
>
> -2. hardlockup_detector_event_create() indirectly calls
> kmem_cache_alloc_node(), which is blockable.
>
> So here, spelling out the really planned context "is_percpu_thread()".
The description is pretty hard to understand. I would suggest
something like:
Subject: kernel/watchdog_hld: Ensure CPU-bound context when creating
hardlockup detector event
hardlockup_detector_event_create() should create perf_event on the
current CPU. Preemption could not get disabled because
perf_event_create_kernel_counter() allocates memory. Instead,
the CPU locality is achieved by processing the code in a per-CPU
bound kthread.
Add a check to prevent mistakes when calling the code in another
code path.
> Signed-off-by: Pingfan Liu <kernelfans at gmail.com>
> Cc: Petr Mladek <pmladek at suse.com>
> Cc: Andrew Morton <akpm at linux-foundation.org>
> Cc: Wang Qing <wangqing at vivo.com>
> Cc: "Peter Zijlstra (Intel)" <peterz at infradead.org>
> Cc: Santosh Sivaraj <santosh at fossix.org>
> Cc: linux-arm-kernel at lists.infradead.org
> To: linux-kernel at vger.kernel.org
> ---
> kernel/watchdog_hld.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/watchdog_hld.c b/kernel/watchdog_hld.c
> index 247bf0b1582c..df010df76576 100644
> --- a/kernel/watchdog_hld.c
> +++ b/kernel/watchdog_hld.c
> @@ -165,10 +165,13 @@ static void watchdog_overflow_callback(struct perf_event *event,
>
> static int hardlockup_detector_event_create(void)
> {
> - unsigned int cpu = smp_processor_id();
> + unsigned int cpu;
> struct perf_event_attr *wd_attr;
> struct perf_event *evt;
>
> + /* This function plans to execute in cpu bound kthread */
This does not explain why it is needed. I suggest something like:
/*
* Preemption is not disabled because memory will be allocated.
* Ensure CPU-locality by calling this in per-CPU kthread.
*/
> + WARN_ON(!is_percpu_thread());
> + cpu = raw_smp_processor_id();
> wd_attr = &wd_hw_attr;
> wd_attr->sample_period = hw_nmi_get_sample_period(watchdog_thresh);
>
Othrewise the change looks good to me.
Best Regards,
Petr
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