• Daniel Thompson's avatar
    ARM: 8255/1: perf: Prevent wraparound during overflow · 2d9ed740
    Daniel Thompson authored
    If the overflow threshold for a counter is set above or near the
    0xffffffff boundary then the kernel may lose track of the overflow
    causing only events that occur *after* the overflow to be recorded.
    Specifically the problem occurs when the value of the performance counter
    overtakes its original programmed value due to wrap around.
    
    Typical solutions to this problem are either to avoid programming in
    values likely to be overtaken or to treat the overflow bit as the 33rd
    bit of the counter.
    
    Its somewhat fiddly to refactor the code to correctly handle the 33rd bit
    during irqsave sections (context switches for example) so instead we take
    the simpler approach of avoiding values likely to be overtaken.
    
    We set the limit to half of max_period because this matches the limit
    imposed in __hw_perf_event_init(). This causes a doubling of the interrupt
    rate for large threshold values, however even with a very fast counter
    ticking at 4GHz the interrupt rate would only be ~1Hz.
    Signed-off-by: default avatarDaniel Thompson <daniel.thompson@linaro.org>
    Acked-by: default avatarWill Deacon <will.deacon@arm.com>
    Signed-off-by: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
    2d9ed740
perf_event.c 12.9 KB