• Paul Mackerras's avatar
    powerpc: Account time using timebase rather than PURR · cf9efce0
    Paul Mackerras authored
    
    
    Currently, when CONFIG_VIRT_CPU_ACCOUNTING is enabled, we use the
    PURR register for measuring the user and system time used by
    processes, as well as other related times such as hardirq and
    softirq times.  This turns out to be quite confusing for users
    because it means that a program will often be measured as taking
    less time when run on a multi-threaded processor (SMT2 or SMT4 mode)
    than it does when run on a single-threaded processor (ST mode), even
    though the program takes longer to finish.  The discrepancy is
    accounted for as stolen time, which is also confusing, particularly
    when there are no other partitions running.
    
    This changes the accounting to use the timebase instead, meaning that
    the reported user and system times are the actual number of real-time
    seconds that the program was executing on the processor thread,
    regardless of which SMT mode the processor is in.  Thus a program will
    generally show greater user and system times when run on a
    multi-threaded processor than on a single-threaded processor.
    
    On pSeries systems on POWER5 or later processors, we measure the
    stolen time (time when this partition wasn't running) using the
    hypervisor dispatch trace log.  We check for new entries in the
    log on every entry from user mode and on every transition from
    kernel process context to soft or hard IRQ context (i.e. when
    account_system_vtime() gets called).  So that we can correctly
    distinguish time stolen from user time and time stolen from system
    time, without having to check the log on every exit to user mode,
    we store separate timestamps for exit to user mode and entry from
    user mode.
    
    On systems that have a SPURR (POWER6 and POWER7), we read the SPURR
    in account_system_vtime() (as before), and then apportion the SPURR
    ticks since the last time we read it between scaled user time and
    scaled system time according to the relative proportions of user
    time and system time over the same interval.  This avoids having to
    read the SPURR on every kernel entry and exit.  On systems that have
    PURR but not SPURR (i.e., POWER5), we do the same using the PURR
    rather than the SPURR.
    
    This disables the DTL user interface in /sys/debug/kernel/powerpc/dtl
    for now since it conflicts with the use of the dispatch trace log
    by the time accounting code.
    Signed-off-by: default avatarPaul Mackerras <paulus@samba.org>
    Signed-off-by: default avatarBenjamin Herrenschmidt <benh@kernel.crashing.org>
    cf9efce0
lppaca.h 8.24 KB