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Subject: Re: [PATCH v4] sched/debug: Use sched_debug_lock to serialize use of
 cgroup_path[] only
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Bharata B Rao <bharata@linux.vnet.ibm.com>,
        Phil Auld <pauld@redhat.com>,
        Daniel Thompson <daniel.thompson@linaro.org>,
        linux-kernel@vger.kernel.org
References: <20210405234203.23526-1-longman@redhat.com>
 <20210405201807.4ee7778d@gandalf.local.home>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <5c4d0d47-5503-76a2-0f27-1b4b13c34aa5@redhat.com>
Date:   Mon, 5 Apr 2021 21:57:07 -0400
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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