Patch regressing rr has landed on Linux kernel stable branches

Robert O'Callahan robert at ocallahan.org
Thu Jun 29 23:25:59 UTC 2017


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.

Rob
-- 
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lurpr
.a war hsrer holsa rodvted,t  nenh hneireseoouot.tniesiewaoeivatewt sstvr
esn
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