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    SubjectRe: [PATCH 2/2] cpuhotplug: make get_online_cpus() scalability by using percpu counter
    Peter Zijlstra wrote:
    > On Mon, 2010-04-12 at 17:24 +0800, Lai Jiangshan wrote:
    >> Oleg Nesterov wrote:
    >>> On 04/07, Oleg Nesterov wrote:
    >>>> On 04/07, Lai Jiangshan wrote:
    >>>>> Old get_online_cpus() is read-preference, I think the goal of this ability
    >>>>> is allow get_online_cpus()/put_online_cpus() to be called nested.
    >>>> Sure, I understand why you added task_struct->get_online_cpus_nest.
    >>>>> and use per-task counter for allowing get_online_cpus()/put_online_cpus()
    >>>>> to be called nested, I think this deal is absolutely worth.
    >>>> As I said, I am not going to argue. I can't justify this tradeoff.
    >>> But, I must admit, I'd like to avoid adding the new member to task_struct.
    >>> What do you think about the code below?
    >>> I didn't even try to compile it, just to explain what I mean.
    >>> In short: we have the per-cpu fast counters, plus the slow counter
    >>> which is only used when cpu_hotplug_begin() is in progress.
    >>> Oleg.
    >> get_online_cpus() in your code is still read-preference.
    >> I wish we quit this ability of get_online_cpus().
    > Why?

    Because read-preference RWL will cause write site starvation.

    A user run the following code will cause cpuhotplug starvation.
    (100 processes run sched_setaffinity().)


    #define _GNU_SOURCE
    #include <sched.h>
    #include <sys/types.h>
    #include <unistd.h>
    #include <stdlib.h>

    #define NCPU 4
    #define NPROCESS 100

    cpu_set_t set;
    pid_t target;

    void stress_test(void)
    int cpu;

    for (;;) {
    cpu = rand() % NCPU;
    CPU_SET(cpu, &set);
    sched_setaffinity(target, sizeof(set), &set);
    CPU_CLR(cpu, &set);

    int main(int argc, char *argv[])
    pid_t ret;
    int i;

    target = getpid();
    for (i = 1; i < NPROCESS; i++) {
    ret = fork();
    if (ret < 0)
    else if (ret)
    target = ret;


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