Subject: Re: [PATCH 09/10] Define PERF_PMU_TXN_READ interface

From: Sukadev Bhattiprolu <[email-masked]>
Date: Thu, 13 Aug 2015 13:04:28 -0700

Peter Zijlstra [[email-masked]] wrote:
| On Tue, Aug 11, 2015 at 09:14:00PM -0700, Sukadev Bhattiprolu wrote:
| > | +static void __perf_read_group_add(struct perf_event *leader, u64 read_format, u64 *values)
| > |  {
| > | +	struct perf_event *sub;
| > | +	int n = 1; /* skip @nr */
| > 
| > This n = 1 is to skip over the values[0] = 1 + nr_siblings in the
| > caller.
| > 
| > Anyway, in __perf_read_group_add() we always start with n = 1, however
| > ...
| > | 
| > | +	perf_event_read(leader, true);
| > | +
| > | +	/*
| > | +	 * Since we co-schedule groups, {enabled,running} times of siblings
| > | +	 * will be identical to those of the leader, so we only publish one
| > | +	 * set.
| > | +	 */
| > | +	if (read_format & PERF_FORMAT_TOTAL_TIME_ENABLED) {
| > | +		values[n++] += leader->total_time_enabled +
| > | +			atomic64_read(leader->child_total_time_enabled);
| 
| Note how this is an in-place addition,

Ah, yes, Sorry I missed that. It make sense now and my tests seem to
be running fine.

| 
| > | +	}
| > | 
| > | +	if (read_format & PERF_FORMAT_TOTAL_TIME_RUNNING) {
| > | +		values[n++] += leader->total_time_running +
| > | +			atomic64_read(leader->child_total_time_running);
| 
| and here,
| 
| > | +	}
| > | 
| > | +	/*
| > | +	 * Write {count,id} tuples for every sibling.
| > | +	 */
| > | +	values[n++] += perf_event_count(leader);
| 
| and here,
| 
| 
| > |  	if (read_format & PERF_FORMAT_ID)
| > |  		values[n++] = primary_event_id(leader);
| 
| and this will always assign the same value.
| 
| > | +	list_for_each_entry(sub, &leader->sibling_list, group_entry) {
| > | +		values[n++] += perf_event_count(sub);
| > | +		if (read_format & PERF_FORMAT_ID)
| > | +			values[n++] = primary_event_id(sub);
| 
| Same for these, therefore,
| 
| > | +	}
| > | +}
| > | 
| > | +static int perf_read_group(struct perf_event *event,
| > | +				   u64 read_format, char __user *buf)
| > | +{
| > | +	struct perf_event *leader = event->group_leader, *child;
| > | +	struct perf_event_context *ctx = leader->ctx;
| > | +	int ret = leader->read_size;

One other question, We return leader->read_size but allocate/copy_to_user
the sibling's event->read_size. We consistently use read_format from the
'event' being read, rather than its 'group_leader', so we are ok in terms
of what we copy into values[] for each event in the group.

But, can the leader's read_format (and hence its read_size) differ from
its sibling's read_size? If so, in the current code, we return the event's
read_size but in the new code, we return the leader's read_size.

| > | +	u64 *values;
| > | 
| > | +	lockdep_assert_held(&ctx->mutex);
| > | 
| > | +	values = kzalloc(event->read_size);
| > | +	if (!values)
| > | +		return -ENOMEM;
| > | 
| > | +	values[0] = 1 + leader->nr_siblings;
| > | 
| > | +	/*
| > | +	 * By locking the child_mutex of the leader we effectively
| > | +	 * lock the child list of all siblings.. XXX explain how.
| > | +	 */
| > | +	mutex_lock(&leader->child_mutex);
| > | 
| > | +	__perf_read_group_add(leader, read_format, values);
| > 
| > ... we don't copy_to_user() here,
| > 
| > | +	list_for_each_entry(child, &leader->child_list, child_list)
| > | +		__perf_read_group_add(child, read_format, values);
| > 
| > so won't we overwrite the values[], if we always start at n = 1
| > in __perf_read_group_add()?
| 
| yes and no, we have to re-iterate the same values for each child as they
| all have the same group, but we add the time and count fields, we do not
| overwrite. The _add() suffix was supposed to be a hint ;-)
| 
| > | +	mutex_unlock(&leader->child_mutex);
| > | +
| > | +	if (copy_to_user(buf, values, event->read_size))
| > | +		ret = -EFAULT;
| > | +
| > | +	kfree(values);
| > | 
| > |  	return ret;
| > |  }
| 
| Where previously we would iterate the group and for each member
| iterate/sum all the child values together before copying the value out,
| we now, because we need to read groups together, need to first iterate
| the child list and sum whole groups.