• Milind Chabbi's avatar
    perf/core: Implement fast breakpoint modification via _IOC_MODIFY_ATTRIBUTES · 32ff77e8
    Milind Chabbi authored
    Problem and motivation: Once a breakpoint perf event (PERF_TYPE_BREAKPOINT)
    is created, there is no flexibility to change the breakpoint type
    (bp_type), breakpoint address (bp_addr), or breakpoint length (bp_len). The
    only option is to close the perf event and configure a new breakpoint
    event. This inflexibility has a significant performance overhead. For
    example, sampling-based, lightweight performance profilers (and also
    concurrency bug detection tools),  monitor different addresses for a short
    duration using PERF_TYPE_BREAKPOINT and change the address (bp_addr) to
    another address or change the kind of breakpoint (bp_type) from  "write" to
    a "read" or vice-versa or change the length (bp_len) of the address being
    monitored. The cost of these modifications is prohibitive since it involves
    unmapping the circular buffer associated with the perf event, closing the
    perf event, opening another perf event and mmaping another circular buffer.
    Solution: The new ioctl flag for perf events,
    PERF_EVENT_IOC_MODIFY_ATTRIBUTES, introduced in this patch takes a pointer
    to a struct perf_event_attr as an argument to update an old breakpoint
    event with new address, type, and size. This facility allows retaining a
    previous mmaped perf events ring buffer and avoids having to close and
    reopen another perf event.
    This patch supports only changing PERF_TYPE_BREAKPOINT event type; future
    implementations can extend this feature. The patch replicates some of its
    functionality of modify_user_hw_breakpoint() in
    kernel/events/hw_breakpoint.c. modify_user_hw_breakpoint cannot be called
    directly since perf_event_ctx_lock() is already held in _perf_ioctl().
    Evidence: Experiments show that the baseline (not able to modify an already
    created breakpoint) costs an order of magnitude (~10x) more than the
    suggested optimization (having the ability to dynamically modifying a
    configured breakpoint via ioctl). When the breakpoints typically do not
    trap, the speedup due to the suggested optimization is ~10x; even when the
    breakpoints always trap, the speedup is ~4x due to the suggested
    Testing: tests posted at
    https://github.com/linux-contrib/perf_event_modify_bp demonstrate the
    performance significance of this patch. Tests also check the functional
    correctness of the patch.
    Signed-off-by: default avatarMilind Chabbi <chabbi.milind@gmail.com>
    [ Using modify_user_hw_breakpoint_check function. ]
    [ Reformated PERF_EVENT_IOC_*, so the values are all in one column. ]
    Signed-off-by: default avatarJiri Olsa <jolsa@kernel.org>
    Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
    Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
    Cc: David Ahern <dsahern@gmail.com>
    Cc: Frederic Weisbecker <fweisbec@gmail.com>
    Cc: Hari Bathini <hbathini@linux.vnet.ibm.com>
    Cc: Jin Yao <yao.jin@linux.intel.com>
    Cc: Jiri Olsa <jolsa@redhat.com>
    Cc: Kan Liang <kan.liang@intel.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Michael Ellerman <mpe@ellerman.id.au>
    Cc: Namhyung Kim <namhyung@kernel.org>
    Cc: Oleg Nesterov <onestero@redhat.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Will Deacon <will.deacon@arm.com>
    Link: http://lkml.kernel.org/r/20180312134548.31532-8-jolsa@kernel.orgSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
hw_breakpoint.h 4.1 KB