commit 247110bb9e52876f0412ece815c81ca055a3a898
author Sami Kyostila <skyostil@google.com> Wed Feb 14 10:18:45 2018 +0000
committer Sami Kyostila <skyostil@google.com> Wed Feb 14 10:18:45 2018 +0000
tree 8bdcedb55b942f98adac2ffd03468d68f879756c
parent 6eb0ee5b8c77305ea8da796f76a5199cfdd0c0e6

Improve ftrace scheduling

This patch implements the following ftrace scheduling algorithm:


  main thread                           [drain] [unblock]
                                        /:              |
                            post .-----' :              |
                                /        :              v
  worker #0  [splice ...] [wakeup] [block ............] [splice]
                                         :
  worker #1  [splice ...]     [wakeup] [block ........] [splice]
                                         :
  worker #2  [splice ..........................................]
                                         :
                                         :
                                    drain period (100ms)

In other words, the splice(2) system call is used to move data from
the raw kernel ftrace pipe into an intermediate pipe at a page
granularity. This call allows every per-cpu worker to sleep until there
is at least one page of data available.

When a worker wakes up, it will attempt to move as many pages as
possible to its staging pipe (up to 64K, depending on the
system's pipe buffer size) in a non-blocking way. After this, it
will notify the main thread that data is available. This notification
will block the calling worker until the main thread has drained the
data.

When at least one worker has woken up, we schedule a drain operation
on the main thread for the next drain period (every 100ms by default).
The drain operation parses ftrace data from the staging pipes of
every worker having pending data. After this, each waiting worker is
allowed to issue another call to splice(), restarting the cycle.

Change-Id: I9657a5d50cd936f190bc57f2c9778019cb7419ab