Re: [PATCH v2]: perf/core: addressing 4x slowdown during per-process, profiling of STREAM benchmark on Intel Xeon Phi

[Peter Zijlstra]

On Mon, May 29, 2017 at 12:24:53PM +0300, Alexey Budankov wrote:
> On 29.05.2017 10:45, Peter Zijlstra wrote:
> > On Sat, May 27, 2017 at 02:19:51PM +0300, Alexey Budankov wrote:
> > > Solution:
> > > 
> > > cpu indexed trees for perf_event_context::pinned_groups and
> > > perf_event_context::flexible_groups lists are introduced. Every tree node
> > > keeps a list of groups allocated for the same cpu. A tree references only
> > > groups located at the appropriate group list. The tree provides capability
> > > to iterate over groups allocated for a specific cpu only, what is exactly
> > > required by multiplexing timer interrupt handler. The handler runs per-cpu
> > > and enables/disables groups using group_sched_in()/group_sched_out() that
> > > call event_filter_match() function filtering out groups allocated for cpus
> > > different from the one executing the handler. Additionally for every
> > > filtered out group group_sched_out() updates tstamps values to the current
> > > interrupt time. This updating work is now done only once by
> > > update_context_time() called by ctx_sched_out() before cpu groups
> > > iteration. For this trick to work it is required that tstamps of filtered
> > > out events would point to perf_event_context::tstamp_data object instead
> > > of perf_event::tstamp_data ones, as it is initialized from an event
> > > allocation. tstamps references are switched by
> > > group_sched_in()/group_sched_out() every time a group is checked for its
> > > suitability for currently running cpu. When a thread enters some cpu on
> > > a context switch a long run through pinned and flexible groups is
> > > performed by perf_event_sched_in(, mux=0) with new parameter mux set to 0
> > > and filtered out groups tstamps are switched to
> > > perf_event_context::tstamp_data object. Then a series of multiplexing
> > > interrupts happens and the handler rotates the flexible groups calling
> > > ctx_sched_out(,mux=1)/perf_event_sched_in(,mux=1) iterating over the cpu
> > > tree lists only and avoiding long runs through the complete group lists.
> > > This is where speedup comes from. Eventually when the thread leaves the cpu
> > > ctx_sched_out(,mux=0) is called restoring tstamps pointers to the events'
> > > perf_event::tstamp_data objects.
> > 
> > This is unreadable.. Please use whitespace.
> 
> Do you mean do NOT use whitespaces? Could you explain in more detail what
> you mean?

No, add _more_ whitespace. Use things like paragraphs and such. Reading
a massive blob of text like that is painful. Also use full and complete
sentences. For example, the very first sentence:

 'cpu indexed trees for perf_event_context::pinned_groups and
 perf_event_context::flexible_groups lists are introduced.'

feels incomplete and leaves one wondering what for etc.. And it only
gets worse.


> > Yeah, this doesn't go into a changelog. Have you _ever_ seen a changelog
> > with such crud in?
> > 
> 
> I have not. Could you please advise how to format this information to be
> suitable for changelog, if it's required at all?

Don't include it. It should be fairly obvious from the diff itself what
changed after all.