• Frank Mayhar's avatar
    timers: fix itimer/many thread hang · f06febc9
    Frank Mayhar authored
    This patch reworks the handling of POSIX CPU timers, including the
    ITIMER_PROF, ITIMER_VIRT timers and rlimit handling.  It was put together
    with the help of Roland McGrath, the owner and original writer of this code.
    The problem we ran into, and the reason for this rework, has to do with using
    a profiling timer in a process with a large number of threads.  It appears
    that the performance of the old implementation of run_posix_cpu_timers() was
    at least O(n*3) (where "n" is the number of threads in a process) or worse.
    Everything is fine with an increasing number of threads until the time taken
    for that routine to run becomes the same as or greater than the tick time, at
    which point things degrade rather quickly.
    This patch fixes bug 9906, "Weird hang with NPTL and SIGPROF."
    Code Changes
    This rework corrects the implementation of run_posix_cpu_timers() to make it
    run in constant time for a particular machine.  (Performance may vary between
    one machine and another depending upon whether the kernel is built as single-
    or multiprocessor and, in the latter case, depending upon the number of
    running processors.)  To do this, at each tick we now update fields in
    signal_struct as well as task_struct.  The run_posix_cpu_timers() function
    uses those fields to make its decisions.
    We define a new structure, "task_cputime," to contain user, system and
    scheduler times and use these in appropriate places:
    struct task_cputime {
    	cputime_t utime;
    	cputime_t stime;
    	unsigned long long sum_exec_runtime;
    This is included in the structure "thread_group_cputime," which is a new
    substructure of signal_struct and which varies for uniprocessor versus
    multiprocessor kernels.  For uniprocessor kernels, it uses "task_cputime" as
    a simple substructure, while for multiprocessor kernels it is a pointer:
    struct thread_group_cputime {
    	struct task_cputime totals;
    struct thread_group_cputime {
    	struct task_cputime *totals;
    We also add a new task_cputime substructure directly to signal_struct, to
    cache the earliest expiration of process-wide timers, and task_cputime also
    replaces the it_*_expires fields of task_struct (used for earliest expiration
    of thread timers).  The "thread_group_cputime" structure contains process-wide
    timers that are updated via account_user_time() and friends.  In the non-SMP
    case the structure is a simple aggregator; unfortunately in the SMP case that
    simplicity was not achievable due to cache-line contention between CPUs (in
    one measured case performance was actually _worse_ on a 16-cpu system than
    the same test on a 4-cpu system, due to this contention).  For SMP, the
    thread_group_cputime counters are maintained as a per-cpu structure allocated
    using alloc_percpu().  The timer functions update only the timer field in
    the structure corresponding to the running CPU, obtained using per_cpu_ptr().
    We define a set of inline functions in sched.h that we use to maintain the
    thread_group_cputime structure and hide the differences between UP and SMP
    implementations from the rest of the kernel.  The thread_group_cputime_init()
    function initializes the thread_group_cputime structure for the given task.
    The thread_group_cputime_alloc() is a no-op for UP; for SMP it calls the
    out-of-line function thread_group_cputime_alloc_smp() to allocate and fill
    in the per-cpu structures and fields.  The thread_group_cputime_free()
    function, also a no-op for UP, in SMP frees the per-cpu structures.  The
    thread_group_cputime_clone_thread() function (also a UP no-op) for SMP calls
    thread_group_cputime_alloc() if the per-cpu structures haven't yet been
    allocated.  The thread_group_cputime() function fills the task_cputime
    structure it is passed with the contents of the thread_group_cputime fields;
    in UP it's that simple but in SMP it must also safely check that tsk->signal
    is non-NULL (if it is it just uses the appropriate fields of task_struct) and,
    if so, sums the per-cpu values for each online CPU.  Finally, the three
    functions account_group_user_time(), account_group_system_time() and
    account_group_exec_runtime() are used by timer functions to update the
    respective fields of the thread_group_cputime structure.
    Non-SMP operation is trivial and will not be mentioned further.
    The per-cpu structure is always allocated when a task creates its first new
    thread, via a call to thread_group_cputime_clone_thread() from copy_signal().
    It is freed at process exit via a call to thread_group_cputime_free() from
    All functions that formerly summed utime/stime/sum_sched_runtime values from
    from all threads in the thread group now use thread_group_cputime() to
    snapshot the values in the thread_group_cputime structure or the values in
    the task structure itself if the per-cpu structure hasn't been allocated.
    Finally, the code in kernel/posix-cpu-timers.c has changed quite a bit.
    The run_posix_cpu_timers() function has been split into a fast path and a
    slow path; the former safely checks whether there are any expired thread
    timers and, if not, just returns, while the slow path does the heavy lifting.
    With the dedicated thread group fields, timers are no longer "rebalanced" and
    the process_timer_rebalance() function and related code has gone away.  All
    summing loops are gone and all code that used them now uses the
    thread_group_cputime() inline.  When process-wide timers are set, the new
    task_cputime structure in signal_struct is used to cache the earliest
    expiration; this is checked in the fast path.
    The fix appears not to add significant overhead to existing operations.  It
    generally performs the same as the current code except in two cases, one in
    which it performs slightly worse (Case 5 below) and one in which it performs
    very significantly better (Case 2 below).  Overall it's a wash except in those
    two cases.
    I've since done somewhat more involved testing on a dual-core Opteron system.
    Case 1: With no itimer running, for a test with 100,000 threads, the fixed
    	kernel took 1428.5 seconds, 513 seconds more than the unfixed system,
    	all of which was spent in the system.  There were twice as many
    	voluntary context switches with the fix as without it.
    Case 2: With an itimer running at .01 second ticks and 4000 threads (the most
    	an unmodified kernel can handle), the fixed kernel ran the test in
    	eight percent of the time (5.8 seconds as opposed to 70 seconds) and
    	had better tick accuracy (.012 seconds per tick as opposed to .023
    	seconds per tick).
    Case 3: A 4000-thread test with an initial timer tick of .01 second and an
    	interval of 10,000 seconds (i.e. a timer that ticks only once) had
    	very nearly the same performance in both cases:  6.3 seconds elapsed
    	for the fixed kernel versus 5.5 seconds for the unfixed kernel.
    With fewer threads (eight in these tests), the Case 1 test ran in essentially
    the same time on both the modified and unmodified kernels (5.2 seconds versus
    5.8 seconds).  The Case 2 test ran in about the same time as well, 5.9 seconds
    versus 5.4 seconds but again with much better tick accuracy, .013 seconds per
    tick versus .025 seconds per tick for the unmodified kernel.
    Since the fix affected the rlimit code, I also tested soft and hard CPU limits.
    Case 4: With a hard CPU limit of 20 seconds and eight threads (and an itimer
    	running), the modified kernel was very slightly favored in that while
    	it killed the process in 19.997 seconds of CPU time (5.002 seconds of
    	wall time), only .003 seconds of that was system time, the rest was
    	user time.  The unmodified kernel killed the process in 20.001 seconds
    	of CPU (5.014 seconds of wall time) of which .016 seconds was system
    	time.  Really, though, the results were too close to call.  The results
    	were essentially the same with no itimer running.
    Case 5: With a soft limit of 20 seconds and a hard limit of 2000 seconds
    	(where the hard limit would never be reached) and an itimer running,
    	the modified kernel exhibited worse tick accuracy than the unmodified
    	kernel: .050 seconds/tick versus .028 seconds/tick.  Otherwise,
    	performance was almost indistinguishable.  With no itimer running this
    	test exhibited virtually identical behavior and times in both cases.
    In times past I did some limited performance testing.  those results are below.
    On a four-cpu Opteron system without this fix, a sixteen-thread test executed
    in 3569.991 seconds, of which user was 3568.435s and system was 1.556s.  On
    the same system with the fix, user and elapsed time were about the same, but
    system time dropped to 0.007 seconds.  Performance with eight, four and one
    thread were comparable.  Interestingly, the timer ticks with the fix seemed
    more accurate:  The sixteen-thread test with the fix received 149543 ticks
    for 0.024 seconds per tick, while the same test without the fix received 58720
    for 0.061 seconds per tick.  Both cases were configured for an interval of
    0.01 seconds.  Again, the other tests were comparable.  Each thread in this
    test computed the primes up to 25,000,000.
    I also did a test with a large number of threads, 100,000 threads, which is
    impossible without the fix.  In this case each thread computed the primes only
    up to 10,000 (to make the runtime manageable).  System time dominated, at
    1546.968 seconds out of a total 2176.906 seconds (giving a user time of
    629.938s).  It received 147651 ticks for 0.015 seconds per tick, still quite
    accurate.  There is obviously no comparable test without the fix.
    Signed-off-by: default avatarFrank Mayhar <fmayhar@google.com>
    Cc: Roland McGrath <roland@redhat.com>
    Cc: Alexey Dobriyan <adobriyan@gmail.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
fork.c 40.7 KB