Re: [PATCH V8 02/12] perf/x86/intel/pt: Add support for pause / resume

[Adrian Hunter]

On 1/07/24 13:46, Peter Zijlstra wrote:
> On Fri, Jun 28, 2024 at 09:51:01AM +0300, Adrian Hunter wrote:
> 
>> @@ -534,7 +537,20 @@ static void pt_config(struct perf_event *event)
>>  	reg |= (event->attr.config & PT_CONFIG_MASK);
>>  
>>  	event->hw.config = reg;
>> +
>> +	/*
>> +	 * Allow resume before starting so as not to overwrite a value set by a
>> +	 * PMI.
>> +	 */
>> +	WRITE_ONCE(pt->resume_allowed, 1);
> 	barrier();
>>  	pt_config_start(event);
> 	barrier();
>> +	/*
>> +	 * Allow pause after starting so its pt_config_stop() doesn't race with
>> +	 * pt_config_start().
>> +	 */
>> +	WRITE_ONCE(pt->pause_allowed, 1);
> 
> IIRC you need those barrier()s, because if the compiler 'helpfully'
> inlines the static pt_config_start(), you loose your sequence point and
> things can get re-ordered. WRITE_ONCE() only ensures the store is whole
> and ordered against other volatile ops, but not against much else.

Yes barriers are needed.  gcc even says:

	...an implementation is free to reorder and combine volatile
	accesses that occur between sequence points...

	Accesses to non-volatile objects are not ordered with respect
	to volatile accesses.

	https://gcc.gnu.org/onlinedocs/gcc/Volatiles.html

> 
>>  }
>>  
>>  static void pt_config_stop(struct perf_event *event)
>> @@ -1511,6 +1527,7 @@ void intel_pt_interrupt(void)
>>  		buf = perf_aux_output_begin(&pt->handle, event);
>>  		if (!buf) {
>>  			event->hw.state = PERF_HES_STOPPED;
>> +			pt->resume_allowed = 0;
>>  			return;
>>  		}
>>  
>> @@ -1519,6 +1536,7 @@ void intel_pt_interrupt(void)
>>  		ret = pt_buffer_reset_markers(buf, &pt->handle);
>>  		if (ret) {
>>  			perf_aux_output_end(&pt->handle, 0);
>> +			pt->resume_allowed = 0;
>>  			return;
>>  		}
>>  
> 
> Above you WRITE_ONCE() on ->resume_allowed, here you do not. Some *SAN
> thing or other is bound to get upset about things like that.
> 
>> @@ -1573,6 +1591,26 @@ static void pt_event_start(struct perf_event *event, int mode)
>>  	struct pt *pt = this_cpu_ptr(&pt_ctx);
>>  	struct pt_buffer *buf;
>>  
>> +	if (mode & PERF_EF_RESUME) {
>> +		if (READ_ONCE(pt->resume_allowed)) {
> 
> At this point I seem to have lost the plot, how do ->resume_allowed and
> ->aux_paused interact?

This is assuming there is no guaranteed order that events
in the group are started / stopped, so there is a need to
protect against pt being resumed when it is not yet configured,
or when it is in the process of being stopped.

Updates to event->hw.state don't happen at quite the right
places to be used instead, and the needs of resume are slightly
different to pause.

resume_allowed means pt is configured and so ->start(PERF_EF_RESUME)
can actually safely start pt. aux_paused is separate and takes
effect when there is a regular start.

> 
>> +			u64 status;
>> +
>> +			/*
>> +			 * Only if the trace is not active and the error and
>> +			 * stopped bits are clear, is it safe to start, but a
>> +			 * PMI might have just cleared these, so resume_allowed
>> +			 * must be checked again also.
>> +			 */
>> +			rdmsrl(MSR_IA32_RTIT_STATUS, status);
>> +			if (!(status & (RTIT_STATUS_TRIGGEREN |
>> +					RTIT_STATUS_ERROR |
>> +					RTIT_STATUS_STOPPED)) &&
>> +			   READ_ONCE(pt->resume_allowed))
>> +				pt_config_start(event);
>> +		}
>> +		return;
>> +	}
>> +
>>  	buf = perf_aux_output_begin(&pt->handle, event);
>>  	if (!buf)
>>  		goto fail_stop;
>> @@ -1601,6 +1639,16 @@ static void pt_event_stop(struct perf_event *event, int mode)
>>  {
>>  	struct pt *pt = this_cpu_ptr(&pt_ctx);
>>  
>> +	if (mode & PERF_EF_PAUSE) {
>> +		if (READ_ONCE(pt->pause_allowed))
>> +			pt_config_stop(event);
>> +		return;
>> +	}
>> +
>> +	/* Protect against racing */
> 
> No F1 cars allowed? Sure the comment can elucidate the reader as to what
> actual race one is concerned about, no?

Yeah this needs more comments.

> 
>> +	WRITE_ONCE(pt->pause_allowed, 0);
>> +	WRITE_ONCE(pt->resume_allowed, 0);
>> +
>>  	/*
>>  	 * Protect against the PMI racing with disabling wrmsr,
>>  	 * see comment in intel_pt_interrupt().
>> @@ -1659,8 +1707,12 @@ static long pt_event_snapshot_aux(struct perf_event *event,
>>  	/*
>>  	 * Here, handle_nmi tells us if the tracing is on
>>  	 */
>> -	if (READ_ONCE(pt->handle_nmi))
>> +	if (READ_ONCE(pt->handle_nmi)) {
>> +		/* Protect against racing */
>> +		WRITE_ONCE(pt->pause_allowed, 0);
>> +		WRITE_ONCE(pt->resume_allowed, 0);
> 
> barrier()?
> 
>>  		pt_config_stop(event);
>> +	}
>>  
>>  	pt_read_offset(buf);
>>  	pt_update_head(pt);
>> @@ -1677,8 +1729,11 @@ static long pt_event_snapshot_aux(struct perf_event *event,
>>  	 * Compiler barrier not needed as we couldn't have been
>>  	 * preempted by anything that touches pt->handle_nmi.
>>  	 */
>> -	if (pt->handle_nmi)
>> +	if (pt->handle_nmi) {
>> +		WRITE_ONCE(pt->resume_allowed, 1);
>>  		pt_config_start(event);
>> +		WRITE_ONCE(pt->pause_allowed, 1);
> 
> barrier() went missing again?
> 
>> +	}
>>  
>>  	return ret;
>>  }