Re: [PATCH 2/2] perf/core: Support reading group events with shared cgroups

[Namhyung Kim]

On Mon, Mar 29, 2021 at 2:32 AM Song Liu <songliubraving@xxxxxx> wrote:
> > On Mar 23, 2021, at 9:21 AM, Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> >
> > This enables reading event group's counter values together with a
> > PERF_EVENT_IOC_READ_CGROUP command like we do in the regular read().
> > Users should give a correct size of buffer to be read.
> >
> > Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> > ---
> > kernel/events/core.c | 119 +++++++++++++++++++++++++++++++++++++++++--
> > 1 file changed, 116 insertions(+), 3 deletions(-)
> >
>
> [...]
>
> > +}
> > +
> > +static int perf_event_read_cgrp_node_group(struct perf_event *event, u64 cgrp_id,
> > +                                        char __user *buf)
> > +{
> > +     struct perf_cgroup_node *cgrp;
> > +     struct perf_event_context *ctx = event->ctx;
> > +     struct perf_event *sibling;
> > +     u64 read_format = event->attr.read_format;
> > +     unsigned long flags;
> > +     u64 *values;
> > +     int n = 1;
> > +     int ret;
> > +
> > +     values = kzalloc(event->read_size, GFP_KERNEL);
> > +     if (!values)
> > +             return -ENOMEM;
> > +
> > +     values[0] = 1 + event->nr_siblings;
> > +
> > +     /* update event count and times (possibly run on other cpu) */
> > +     (void)perf_event_read(event, true);
> > +
> > +     raw_spin_lock_irqsave(&ctx->lock, flags);
> > +
> > +     cgrp = find_cgroup_node(event, cgrp_id);
> > +     if (cgrp == NULL) {
> > +             raw_spin_unlock_irqrestore(&ctx->lock, flags);
> > +             kfree(values);
> > +             return -ENOENT;
> > +     }
> > +
> > +     if (read_format & PERF_FORMAT_TOTAL_TIME_ENABLED)
> > +             values[n++] = cgrp->time_enabled;
> > +     if (read_format & PERF_FORMAT_TOTAL_TIME_RUNNING)
> > +             values[n++] = cgrp->time_running;
> > +
> > +     values[n++] = cgrp->count;
> > +     if (read_format & PERF_FORMAT_ID)
> > +             values[n++] = primary_event_id(event);
> > +
> > +     for_each_sibling_event(sibling, event) {
> > +             n += perf_event_read_cgrp_node_sibling(sibling, read_format,
> > +                                                    cgrp_id, &values[n]);
> > +     }
> > +
> > +     raw_spin_unlock_irqrestore(&ctx->lock, flags);
> > +
> > +     ret = copy_to_user(buf, values, n * sizeof(u64));
> > +     kfree(values);
> > +     if (ret)
> > +             return -EFAULT;
> > +
> > +     return n * sizeof(u64);
> > +}
> > +
> > +static int perf_event_read_cgroup_node(struct perf_event *event, u64 read_size,
> > +                                    u64 cgrp_id, char __user *buf)
> > +{
> > +     u64 read_format = event->attr.read_format;
> > +
> > +     if (read_size < event->read_size + 2 * sizeof(u64))
>
> Why do we need read_size + 2 u64 here?

I should've repeated the following description in the patch 1.

 * PERF_EVENT_IOC_READ_CGROUP - it takes a buffer consists of a 64-bit
     array to get the event counter values.  The first element is size
     of the array in byte, and the second element is a cgroup id to
     read.  The rest is to save the counter value and timings.

Thanks,
Namhyung