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SubjectRe: [PATCH v4] sched/debug: Use sched_debug_lock to serialize use of cgroup_path[] only
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On 4/5/21 8:18 PM, Steven Rostedt wrote:
> On Mon, 5 Apr 2021 19:42:03 -0400
> Waiman Long <longman@redhat.com> wrote:
>
>> +/*
>> + * All the print_cpu() callers from sched_debug_show() will be allowed
>> + * to contend for sched_debug_lock and use group_path[] as their SEQ_printf()
>> + * calls will be much faster. However only one print_cpu() caller from
>> + * sysrq_sched_debug_show() which outputs to the console will be allowed
>> + * to use group_path[]. Another parallel console writer will have to use
>> + * a shorter stack buffer instead. Since the console output will be garbled
>> + * anyway, truncation of some cgroup paths shouldn't be a big issue.
>> + */
>> +#define SEQ_printf_task_group_path(m, tg, fmt...) \
>> +{ \
>> + unsigned long flags; \
>> + int token = m ? TOKEN_NA \
>> + : xchg_acquire(&console_token, TOKEN_NONE); \
>> + \
>> + if (token == TOKEN_NONE) { \
>> + char buf[128]; \
>> + task_group_path(tg, buf, sizeof(buf)); \
>> + SEQ_printf(m, fmt, buf); \
>> + } else { \
>> + spin_lock_irqsave(&sched_debug_lock, flags); \
>> + task_group_path(tg, group_path, sizeof(group_path)); \
>> + SEQ_printf(m, fmt, group_path); \
>> + spin_unlock_irqrestore(&sched_debug_lock, flags); \
>> + if (token == TOKEN_ACQUIRED) \
>> + smp_store_release(&console_token, token); \
>> + } \
>> }
>> #endif
> And you said my suggestion was complex!
>
> I'll let others review this.
>
This patch is actually inspired by your suggestion, though it is
structured differently from your approach. I really want to thank you
for your valuable feedback.

I realized that printing to a sequence file wasn't really a problem,
only printing to console can be problematic. That is why I decided to
allow unlimited use of group_path[] for those users and only one console
writer is allowed to use it. As for calling touch_nmi_watchdog(), I am
still thinking where will be the right place to do it, but it can be
done with a separate patch, if needed.

Cheers,
Longman

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