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b5711042a1
Currently lock contention timestamp is maintained in a hash map keyed by pid. That means it needs to get and release a map element (which is proctected by spinlock!) on each contention begin and end pair. This can impact on performance if there are a lot of contention (usually from spinlocks). It used to go with task local storage but it had an issue on memory allocation in some critical paths. Although it's addressed in recent kernels IIUC, the tool should support old kernels too. So it cannot simply switch to the task local storage at least for now. As spinlocks create lots of contention and they disabled preemption during the spinning, it can use per-cpu array to keep the timestamp to avoid overhead in hashmap update and delete. In contention_begin, it's easy to check the lock types since it can see the flags. But contention_end cannot see it. So let's try to per-cpu array first (unconditionally) if it has an active element (lock != 0). Then it should be used and per-task tstamp map should not be used until the per-cpu array element is cleared which means nested spinlock contention (if any) was finished and it nows see (the outer) lock. Signed-off-by: Namhyung Kim <namhyung@kernel.org> Acked-by: Ian Rogers <irogers@google.com> Cc: Hao Luo <haoluo@google.com> Cc: Song Liu <song@kernel.org> Cc: bpf@vger.kernel.org Link: https://lore.kernel.org/r/20231020204741.1869520-3-namhyung@kernel.org
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.