From 5cc906b4b4a510b274113ddb3f88d60644553f79 Mon Sep 17 00:00:00 2001 From: Marco Elver Date: Fri, 5 Nov 2021 13:45:37 -0700 Subject: kfence: add note to documentation about skipping covered allocations Add a note briefly mentioning the new policy about "skipping currently covered allocations if pool close to full." Since this has a notable impact on KFENCE's bug-detection ability on systems with large uptimes, it is worth pointing out the feature. Link: https://lkml.kernel.org/r/20210923104803.2620285-5-elver@google.com Signed-off-by: Marco Elver Reviewed-by: Dmitry Vyukov Acked-by: Alexander Potapenko Cc: Aleksandr Nogikh Cc: Jann Horn Cc: Taras Madan Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- Documentation/dev-tools/kfence.rst | 11 +++++++++++ 1 file changed, 11 insertions(+) (limited to 'Documentation/dev-tools') diff --git a/Documentation/dev-tools/kfence.rst b/Documentation/dev-tools/kfence.rst index 0fbe3308bf37..d45f952986ae 100644 --- a/Documentation/dev-tools/kfence.rst +++ b/Documentation/dev-tools/kfence.rst @@ -269,6 +269,17 @@ tail of KFENCE's freelist, so that the least recently freed objects are reused first, and the chances of detecting use-after-frees of recently freed objects is increased. +If pool utilization reaches 75% (default) or above, to reduce the risk of the +pool eventually being fully occupied by allocated objects yet ensure diverse +coverage of allocations, KFENCE limits currently covered allocations of the +same source from further filling up the pool. The "source" of an allocation is +based on its partial allocation stack trace. A side-effect is that this also +limits frequent long-lived allocations (e.g. pagecache) of the same source +filling up the pool permanently, which is the most common risk for the pool +becoming full and the sampled allocation rate dropping to zero. The threshold +at which to start limiting currently covered allocations can be configured via +the boot parameter ``kfence.skip_covered_thresh`` (pool usage%). + Interface --------- -- cgit v1.2.3-59-g8ed1b From 4f612ed3f748962cbef1316ff3d323e2b9055b6e Mon Sep 17 00:00:00 2001 From: Marco Elver Date: Fri, 5 Nov 2021 13:45:49 -0700 Subject: 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 | je ffffffff812d6003 | ... which, given kfence_allocation_gate is infrequently modified, should be well predicted by most CPUs. Link: https://lkml.kernel.org/r/20211019102524.2807208-2-elver@google.com Signed-off-by: Marco Elver Cc: Alexander Potapenko Cc: Dmitry Vyukov Cc: Jann Horn Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- Documentation/dev-tools/kfence.rst | 12 ++++++++---- lib/Kconfig.kfence | 26 +++++++++++++++----------- 2 files changed, 23 insertions(+), 15 deletions(-) (limited to 'Documentation/dev-tools') diff --git a/Documentation/dev-tools/kfence.rst b/Documentation/dev-tools/kfence.rst index d45f952986ae..ac6b89d1a8c3 100644 --- a/Documentation/dev-tools/kfence.rst +++ b/Documentation/dev-tools/kfence.rst @@ -231,10 +231,14 @@ Guarded allocations are set up based on the sample interval. After expiration of the sample interval, the next allocation through the main allocator (SLAB or SLUB) returns a guarded allocation from the KFENCE object pool (allocation sizes up to PAGE_SIZE are supported). At this point, the timer is reset, and -the next allocation is set up after the expiration of the interval. To "gate" a -KFENCE allocation through the main allocator's fast-path without overhead, -KFENCE relies on static branches via the static keys infrastructure. The static -branch is toggled to redirect the allocation to KFENCE. +the next allocation is set up after the expiration of the interval. + +When using ``CONFIG_KFENCE_STATIC_KEYS=y``, KFENCE allocations are "gated" +through the main allocator's fast-path by relying on static branches via the +static keys infrastructure. The static branch is toggled to redirect the +allocation to KFENCE. Depending on sample interval, target workloads, and +system architecture, this may perform better than the simple dynamic branch. +Careful benchmarking is recommended. KFENCE objects each reside on a dedicated page, at either the left or right page boundaries selected at random. The pages to the left and right of the 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 if KFENCE -config KFENCE_STATIC_KEYS - 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. - config KFENCE_SAMPLE_INTERVAL 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. +config KFENCE_STATIC_KEYS + 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. + config KFENCE_STRESS_TEST_FAULTS int "Stress testing of fault handling and error reporting" if EXPERT default 0 -- cgit v1.2.3-59-g8ed1b