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    Subject[PATCH 3/3] perf: builtin-record: Document and check that mmap_pages must be a power of two.
    Date
    Now that we automatically point users at it, let's provide them some
    guidance so that they hopefully don't just get mysterious EINVAL's
    from the kernel.

    Signed-off-by: Nelson Elhage <nelhage@nelhage.com>
    ---
    tools/perf/Documentation/perf-record.txt | 2 +-
    tools/perf/builtin-record.c | 3 +++
    tools/perf/util/util.h | 11 +++++++++++
    3 files changed, 15 insertions(+), 1 deletions(-)

    diff --git a/tools/perf/Documentation/perf-record.txt b/tools/perf/Documentation/perf-record.txt
    index 5a520f8..2937f7e 100644
    --- a/tools/perf/Documentation/perf-record.txt
    +++ b/tools/perf/Documentation/perf-record.txt
    @@ -89,7 +89,7 @@ OPTIONS

    -m::
    --mmap-pages=::
    - Number of mmap data pages.
    + Number of mmap data pages. Must be a power of two.

    -g::
    --call-graph::
    diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
    index 45b23c1..275c78d 100644
    --- a/tools/perf/builtin-record.c
    +++ b/tools/perf/builtin-record.c
    @@ -560,6 +560,9 @@ static int __cmd_record(int argc, const char **argv)
    if (mmap_pages == UINT_MAX)
    mmap_pages = (512 * 1024) / page_size;

    + if (!is_power_of_2(mmap_pages))
    + die("--mmap_pages/-m value must be a power of two.");
    +
    if (forks) {
    child_pid = fork();
    if (child_pid < 0) {
    diff --git a/tools/perf/util/util.h b/tools/perf/util/util.h
    index 0128906..37be34d 100644
    --- a/tools/perf/util/util.h
    +++ b/tools/perf/util/util.h
    @@ -245,4 +245,15 @@ int readn(int fd, void *buf, size_t size);
    #define _STR(x) #x
    #define STR(x) _STR(x)

    +/*
    + * Determine whether some value is a power of two, where zero is
    + * *not* considered a power of two.
    + */
    +
    +static inline __attribute__((const))
    +bool is_power_of_2(unsigned long n)
    +{
    + return (n != 0 && ((n & (n - 1)) == 0));
    +}
    +
    #endif
    --
    1.7.4.41.g50b8f


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