Re: [RFC PATCH] perf: Add PERF_RECORD_SWITCH to indicate context switches

[Adrian Hunter <adrian.hunter@intel.com>]

On 11/06/15 17:15, Peter Zijlstra wrote:
> On Tue, Jun 09, 2015 at 05:21:10PM +0300, Adrian Hunter wrote:
>> Tracepoints are no good at all for non-privileged users
>> because they need either CAP_SYS_ADMIN or
>> /proc/sys/kernel/perf_event_paranoid <= -1.
>>
>> On the other hand, kernel software events need either
>> CAP_SYS_ADMIN or /proc/sys/kernel/perf_event_paranoid <= 1.
> 
> So while I think it makes sense to allow some tracepoint outside of that
> priv level, IOW have a per tracepoint priv level filter thingy, I don't
> think sched_switch() is one of those because it explicitly exposes
> timing information on other tasks.
> 
>> This new PERF_RECORD_SWITCH event does not have those problems
>> and it also has a couple of other small advantages. It is
>> easier to use because it is an auxiliary event (like mmap,
>> comm and task events) which can be enabled by setting a single
>> bit. It is smaller than sched:sched_switch and easier to parse.
> 
> Right, so the one wee problem I have is that this only provides sched_in
> data, I imagine people might be interested in sched_out as well.

That is not a problem although it would be interesting to know the use-case.
To me it seemed unreasonable to expect to analyze scheduler behaviour
without admin-level privileges since it is inherently an administrative
activity.

> 
> Typically the switch even provides prev and next and thereby is
> complete, but since we're limiting it to the one specific task, we'll
> not have the sched_out data.

That makes sense for completeness, but as I wrote, it would be interesting
to know what someone might actually use that for.

> 
>> @@ -812,6 +813,18 @@ enum perf_event_type {
>>  	 */
>>  	PERF_RECORD_ITRACE_START		= 12,
>>  
>> +	/*
>> +	 *
>> +	 *
>> +	 * struct {
>> +	 *	struct perf_event_header	header;
>> +	 *	u32				pid, tid;
>> +	 *	u64				time;
> 
> all 3 are already part of sample_id.

You have to decide whether you expect to be able to use an event without
sample_id. MMAP and MMAP2 both have pid, tid which are in sample_id, LOST
has id, EXIT and FORK have time, all of the THROTTLE/UNTHROTTLE members are
in sample_id etc. So it currently looks like we expect to be able to use an
event without requiring sample_id.

It doesn't affect my case either way because perf tools always sets
sample_id_all if it can.

> 
>> +	 * 	struct sample_id		sample_id;
>> +	 * };
>> + 	 */
>> +	PERF_RECORD_SWITCH			= 13,
> 
>