Patch regressing rr has landed on Linux kernel stable branches

Kyle Huey me at kylehuey.com
Tue Jul 4 16:58:45 UTC 2017


On Thu, Jun 29, 2017 at 4:25 PM, Robert O'Callahan <robert at ocallahan.org> wrote:
> This commit:
> https://github.com/torvalds/linux/commit/cc1582c231ea
> breaks rr. It landed for 4.12rc5, and we're trying to get it fixed before
> 4.12 is released. Unfortunately it has already been backported to 3.18.57,
> 4.4.72, 4.9.32 and 4.11.5 :-(. Obviously we'll try to get the 4.12 fix also
> backported to those branches, but that will take a little while.
>
> The symptoms are that long, complex replays fail with  "overshot target
> ticks=... by <N>" where N is generally a pretty large number (> 1000). If
> you look in the trace file, the value N will usually be similar to the
> difference between the target ticks and the previous ticks value for that
> task --- i.e. we tried to stop after N ticks but we actually stopped after
> about N*2 ticks. Unfortunately, rr tests don't seem to be affected.

Unfortunately the fix did not land before 4.12 was cut.

- Kyle


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