Bpf next rdtsc #8650
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Bpf next rdtsc #8650
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The function gfpflags_allow_spinning() has a bug that makes it return the opposite result than intended. This could contribute to deadlocks as usage profilerates, for now it was noticed as a performance regression due to try_charge_memcg() not refilling memcg stock when it could. Fix the flipped condition. Fixes: 97769a5 ("mm, bpf: Introduce try_alloc_pages() for opportunistic page allocation") Reported-by: kernel test robot <[email protected]> Signed-off-by: Vlastimil Babka <[email protected]> Signed-off-by: Alexei Starovoitov <[email protected]> Signed-off-by: Andrii Nakryiko <[email protected]> Link: https://lore.kernel.org/bpf/[email protected] Closes: https://lore.kernel.org/oe-lkp/[email protected]
Replace comma between expressions with semicolons. Using a ',' in place of a ';' can have unintended side effects. Although that is not the case here, it is seems best to use ';' unless ',' is intended. Found by inspection. No functional change intended. Compile tested only. Signed-off-by: Chen Ni <[email protected]> Signed-off-by: Andrii Nakryiko <[email protected]> Acked-by: Anton Protopopov <[email protected]> Link: https://lore.kernel.org/bpf/[email protected]
New kfunc to return ARCH-specific timecounter. The main reason to implement this kfunc is to avoid extra overhead of benchmark measurements, which are usually done by a pair of bpf_ktime_get_ns() at the beginnig and at the end of the code block under benchmark. This function doesn't implement conversion to the monotonic clock and saves some CPU cycles because of it. Another point is when it's properly JITed, it saves extra hundreds of cycles by receiving timecounter values in single-digit amount of instructions. The delta values can be translated into nanoseconds using kfunc introduced in the next patch. For x86 BPF JIT converts this kfunc into rdtsc ordered call. Other architectures will get JIT implementation too if supported. The fallback is to call __arch_get_hw_counter(). The simplest use-case is added in 4th patch, where we calculate the time spent by bpf_get_ns_current_pid_tgid() kfunc. More complex example is to use session cookie to store timecounter value at kprobe/uprobe using kprobe.session/uprobe.session, and calculate the difference at kretprobe/uretprobe. Acked-by: Eduard Zingerman <[email protected]> Acked-by: Andrii Nakryiko <[email protected]> Acked-by: Yonghong Song <[email protected]> Signed-off-by: Vadim Fedorenko <[email protected]>
The new helper should be used to convert deltas of values received by bpf_get_cpu_time_counter() into nanoseconds. It is not designed to do full conversion of time counter values to CLOCK_MONOTONIC_RAW nanoseconds and cannot guarantee monotonicity of 2 independent values, but rather to convert the difference of 2 close enough values of CPU timestamp counter into nanoseconds. This function is also JITted into just several instructions and adds as low overhead as possible and perfectly suits benchmark use-cases. Reviewed-by: Eduard Zingerman <[email protected]> Acked-by: Andrii Nakryiko <[email protected]> Signed-off-by: Vadim Fedorenko <[email protected]>
bpf_get_cpu_time_counter() is replaced with rdtsc instruction on x86_64. Add tests to check that JIT works as expected. Acked-by: Eduard Zingerman <[email protected]> Signed-off-by: Vadim Fedorenko <[email protected]>
The selftest provides an example of how to measure the latency of bpf kfunc/helper call using time stamp counter and how to convert measured value into nanoseconds. Signed-off-by: Vadim Fedorenko <[email protected]>
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