path: root/lib
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authorMarco Elver <>2021-11-05 13:45:49 -0700
committerLinus Torvalds <>2021-11-06 13:30:43 -0700
commit4f612ed3f748962cbef1316ff3d323e2b9055b6e (patch)
treee169a92246e0e93b1a4e6ac6be8bccf1e552db1e /lib
parent07e8481d3c38f461d7b79c1d5c9afe013b162b0c (diff)
kfence: default to dynamic branch instead of static keys mode
We have observed that on very large machines with newer CPUs, the static key/branch switching delay is on the order of milliseconds. This is due to the required broadcast IPIs, which simply does not scale well to hundreds of CPUs (cores). If done too frequently, this can adversely affect tail latencies of various workloads. One workaround is to increase the sample interval to several seconds, while decreasing sampled allocation coverage, but the problem still exists and could still increase tail latencies. As already noted in the Kconfig help text, there are trade-offs: at lower sample intervals the dynamic branch results in better performance; however, at very large sample intervals, the static keys mode can result in better performance -- careful benchmarking is recommended. Our initial benchmarking showed that with large enough sample intervals and workloads stressing the allocator, the static keys mode was slightly better. Evaluating and observing the possible system-wide side-effects of the static-key-switching induced broadcast IPIs, however, was a blind spot (in particular on large machines with 100s of cores). Therefore, a major downside of the static keys mode is, unfortunately, that it is hard to predict performance on new system architectures and topologies, but also making conclusions about performance of new workloads based on a limited set of benchmarks. Most distributions will simply select the defaults, while targeting a large variety of different workloads and system architectures. As such, the better default is CONFIG_KFENCE_STATIC_KEYS=n, and re-enabling it is only recommended after careful evaluation. For reference, on x86-64 the condition in kfence_alloc() generates exactly 2 instructions in the kmem_cache_alloc() fast-path: | ... | cmpl $0x0,0x1a8021c(%rip) # ffffffff82d560d0 <kfence_allocation_gate> | je ffffffff812d6003 <kmem_cache_alloc+0x243> | ... which, given kfence_allocation_gate is infrequently modified, should be well predicted by most CPUs. Link: Signed-off-by: Marco Elver <> Cc: Alexander Potapenko <> Cc: Dmitry Vyukov <> Cc: Jann Horn <> Signed-off-by: Andrew Morton <> Signed-off-by: Linus Torvalds <>
Diffstat (limited to 'lib')
1 files changed, 15 insertions, 11 deletions
diff --git a/lib/Kconfig.kfence b/lib/Kconfig.kfence
index e641add33947..912f252a41fc 100644
--- a/lib/Kconfig.kfence
+++ b/lib/Kconfig.kfence
@@ -25,17 +25,6 @@ menuconfig KFENCE
- bool "Use static keys to set up allocations"
- default y
- depends on JUMP_LABEL # To ensure performance, require jump labels
- help
- Use static keys (static branches) to set up KFENCE allocations. Using
- static keys is normally recommended, because it avoids a dynamic
- branch in the allocator's fast path. However, with very low sample
- intervals, or on systems that do not support jump labels, a dynamic
- branch may still be an acceptable performance trade-off.
int "Default sample interval in milliseconds"
default 100
@@ -56,6 +45,21 @@ config KFENCE_NUM_OBJECTS
pages are required; with one containing the object and two adjacent
ones used as guard pages.
+ bool "Use static keys to set up allocations" if EXPERT
+ depends on JUMP_LABEL
+ help
+ Use static keys (static branches) to set up KFENCE allocations. This
+ option is only recommended when using very large sample intervals, or
+ performance has carefully been evaluated with this option.
+ Using static keys comes with trade-offs that need to be carefully
+ evaluated given target workloads and system architectures. Notably,
+ enabling and disabling static keys invoke IPI broadcasts, the latency
+ and impact of which is much harder to predict than a dynamic branch.
+ Say N if you are unsure.
int "Stress testing of fault handling and error reporting" if EXPERT
default 0