• Will Deacon's avatar
    perf/aux: Ensure aux_wakeup represents most recent wakeup index · d9a50b02
    Will Deacon authored
    The aux_watermark member of struct ring_buffer represents the period (in
    terms of bytes) at which wakeup events should be generated when data is
    written to the aux buffer in non-snapshot mode. On hardware that cannot
    generate an interrupt when the aux_head reaches an arbitrary wakeup index
    (such as ARM SPE), the aux_head sampled from handle->head in
    perf_aux_output_{skip,end} may in fact be past the wakeup index. This
    can lead to wakeup slowly falling behind the head. For example, consider
    the case where hardware can only generate an interrupt on a page-boundary
    and the aux buffer is initialised as follows:
    
      // Buffer size is 2 * PAGE_SIZE
      rb->aux_head = rb->aux_wakeup = 0
      rb->aux_watermark = PAGE_SIZE / 2
    
    following the first perf_aux_output_begin call, the handle is
    initialised with:
    
      handle->head = 0
      handle->size = 2 * PAGE_SIZE
      handle->wakeup = PAGE_SIZE / 2
    
    and the hardware will be programmed to generate an interrupt at
    PAGE_SIZE.
    
    When the interrupt is raised, the hardware head will be at PAGE_SIZE,
    so calling perf_aux_output_end(handle, PAGE_SIZE) puts the ring buffer
    into the following state:
    
      rb->aux_head = PAGE_SIZE
      rb->aux_wakeup = PAGE_SIZE / 2
      rb->aux_watermark = PAGE_SIZE / 2
    
    and then the next call to perf_aux_output_begin will result in:
    
      handle->head = handle->wakeup = PAGE_SIZE
    
    for which the semantics are unclear and, for a smaller aux_watermark
    (e.g. PAGE_SIZE / 4), then the wakeup would in fact be behind head at
    this point.
    
    This patch fixes the problem by rounding down the aux_head (as sampled
    from the handle) to the nearest aux_watermark boundary when updating
    rb->aux_wakeup, therefore taking into account any overruns by the
    hardware.
    Reported-by: default avatarMark Rutland <mark.rutland@arm.com>
    Signed-off-by: default avatarWill Deacon <will.deacon@arm.com>
    Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
    Acked-by: default avatarAlexander Shishkin <alexander.shishkin@linux.intel.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: linux-arm-kernel@lists.infradead.org
    Link: http://lkml.kernel.org/r/1502900297-21839-2-git-send-email-will.deacon@arm.comSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
    d9a50b02
internal.h 5.8 KB