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2e22502c08
Fixes STAR 9000953410: "perf callgraph profiling causing RCU stalls" | perf record -g -c 15000 -e cycles /sbin/hackbench | | INFO: rcu_preempt self-detected stall on CPU | 1: (1 GPs behind) idle=609/140000000000002/0 softirq=2914/2915 fqs=603 | Task dump for CPU 1: in-kernel dwarf unwinder has a fast binary lookup and a fallback linear search (which iterates thru each of ~11K entries) thus takes 2 orders of magnitude longer (~3 million cycles vs. 2000). Routines written in hand assembler lack dwarf info (as we don't support assembler CFI pseudo-ops yet) fail the unwinder binary lookup, hit linear search, failing nevertheless in the end. However the linear search is pointless as binary lookup tables are created from it in first place. It is impossible to have binary lookup fail while succeed the linear search. It is pure waste of cycles thus removed by this patch. This manifested as RCU stalls / NMI watchdog splat when running hackbench under perf with callgraph profiling. The triggering condition was perf counter overflowing in routine lacking dwarf info (like memset) leading to patheic 3 million cycle unwinder slow path and by the time it returned new interrupts were already pending (Timer, IPI) and taken rightaway. The original memset didn't make forward progress, system kept accruing more interrupts and more unwinder delayes in a vicious feedback loop, ultimately triggering the NMI diagnostic. Cc: stable@vger.kernel.org Signed-off-by: Vineet Gupta <vgupta@synopsys.com> |
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