# git rev-parse -q --verify 3840cbe24cf060ea05a585ca497814609f5d47d1^{commit}
3840cbe24cf060ea05a585ca497814609f5d47d1
already have revision, skipping fetch
# git checkout -q -f -B kisskb 3840cbe24cf060ea05a585ca497814609f5d47d1
# git clean -qxdf
# < git log -1
# commit 3840cbe24cf060ea05a585ca497814609f5d47d1
# Author: Johannes Weiner <hannes@cmpxchg.org>
# Date:   Thu Oct 3 07:29:05 2024 -0400
# 
#     sched: psi: fix bogus pressure spikes from aggregation race
#     
#     Brandon reports sporadic, non-sensical spikes in cumulative pressure
#     time (total=) when reading cpu.pressure at a high rate. This is due to
#     a race condition between reader aggregation and tasks changing states.
#     
#     While it affects all states and all resources captured by PSI, in
#     practice it most likely triggers with CPU pressure, since scheduling
#     events are so frequent compared to other resource events.
#     
#     The race context is the live snooping of ongoing stalls during a
#     pressure read. The read aggregates per-cpu records for stalls that
#     have concluded, but will also incorporate ad-hoc the duration of any
#     active state that hasn't been recorded yet. This is important to get
#     timely measurements of ongoing stalls. Those ad-hoc samples are
#     calculated on-the-fly up to the current time on that CPU; since the
#     stall hasn't concluded, it's expected that this is the minimum amount
#     of stall time that will enter the per-cpu records once it does.
#     
#     The problem is that the path that concludes the state uses a CPU clock
#     read that is not synchronized against aggregators; the clock is read
#     outside of the seqlock protection. This allows aggregators to race and
#     snoop a stall with a longer duration than will actually be recorded.
#     
#     With the recorded stall time being less than the last snapshot
#     remembered by the aggregator, a subsequent sample will underflow and
#     observe a bogus delta value, resulting in an erratic jump in pressure.
#     
#     Fix this by moving the clock read of the state change into the seqlock
#     protection. This ensures no aggregation can snoop live stalls past the
#     time that's recorded when the state concludes.
#     
#     Reported-by: Brandon Duffany <brandon@buildbuddy.io>
#     Link: https://bugzilla.kernel.org/show_bug.cgi?id=219194
#     Link: https://lore.kernel.org/lkml/20240827121851.GB438928@cmpxchg.org/
#     Fixes: df77430639c9 ("psi: Reduce calls to sched_clock() in psi")
#     Cc: stable@vger.kernel.org
#     Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
#     Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
#     Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
# < /opt/cross/kisskb/korg/gcc-13.1.0-nolibc/hppa-linux/bin/hppa-linux-gcc --version
# < /opt/cross/kisskb/korg/gcc-13.1.0-nolibc/hppa-linux/bin/hppa-linux-ld --version
# < git log --format=%s --max-count=1 3840cbe24cf060ea05a585ca497814609f5d47d1
# make -s -j 32 ARCH=parisc O=/kisskb/build/linus_allnoconfig_parisc-gcc13 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-13.1.0-nolibc/hppa-linux/bin/hppa-linux-  allnoconfig
# < make -s -j 32 ARCH=parisc O=/kisskb/build/linus_allnoconfig_parisc-gcc13 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-13.1.0-nolibc/hppa-linux/bin/hppa-linux-  help
# make -s -j 32 ARCH=parisc O=/kisskb/build/linus_allnoconfig_parisc-gcc13 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-13.1.0-nolibc/hppa-linux/bin/hppa-linux-  olddefconfig
# make -s -j 32 ARCH=parisc O=/kisskb/build/linus_allnoconfig_parisc-gcc13 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-13.1.0-nolibc/hppa-linux/bin/hppa-linux-  
Completed OK
# rm -rf /kisskb/build/linus_allnoconfig_parisc-gcc13
# Build took: 0:00:22.624626