Re: [Xen-devel] [PATCH v12 for-xen-4.5 11/20] x86/VPMU: Interface for setting PMU mode and flags

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 25.09.14 at 21:28, <boris.ostrovsky@xxxxxxxxxx> wrote:
> +static int vpmu_force_context_switch(void)
> +{
> +    unsigned i, j, allbutself_num, mycpu;
> +    static s_time_t start, now;

I don't think "now" needs to be static. In fact it would perhaps better
be moved into the more narrow scope below.

> +    struct tasklet **sync_task;
> +    struct vcpu *curr_vcpu = current;
> +    int ret = 0;
> +
> +    allbutself_num = num_online_cpus() - 1;
> +
> +    sync_task = xzalloc_array(struct tasklet *, allbutself_num);

So while the allocation further down got broken up properly, this
till degenerates to an order-greater-0 allocation for beyond 512
CPUs. I think you'd be better of (ab)using per-CPU data here.

> +    if ( !sync_task )
> +    {
> +        printk(XENLOG_WARNING "vpmu_force_context_switch: out of memory\n");
> +        return -ENOMEM;
> +    }
> +
> +    for ( i = 0; i < allbutself_num; i++ )
> +    {
> +        sync_task[i] = xmalloc(struct tasklet);
> +        if ( sync_task[i] == NULL )
> +        {
> +            printk(XENLOG_WARNING "vpmu_force_context_switch: out of 
> memory\n");
> +            ret = -ENOMEM;
> +            goto out;
> +        }
> +        tasklet_init(sync_task[i], vpmu_sched_checkin, 0);
> +    }
> +
> +    atomic_set(&vpmu_sched_counter, 0);
> +
> +    j = 0;
> +    mycpu = smp_processor_id();
> +    for_each_online_cpu( i )
> +    {
> +        if ( i != mycpu )
> +            tasklet_schedule_on_cpu(sync_task[j++], i);
> +    }
> +
> +    vpmu_save(curr_vcpu);
> +
> +    start = NOW();
> +
> +    /*
> +     * Note that we may fail here if a CPU is hot-plugged while we are
> +     * waiting. We will then time out.
> +     */
> +    while ( atomic_read(&vpmu_sched_counter) != allbutself_num )
> +    {
> +        cpu_relax();
> +
> +        now = NOW();
> +
> +        /* Give up after 5 seconds */
> +        if ( now > start + SECONDS(5) )
> +        {
> +            printk(XENLOG_WARNING
> +                   "vpmu_force_context_switch: failed to sync\n");
> +            ret = -EBUSY;
> +            break;
> +        }
> +
> +        /* Or after 2 milliseconds if need to be preempted */

/* Or after 2 ms (arbitrary value) if need to be preempted. */

> +        if ( (now > start + MILLISECS(2)) && hypercall_preempt_check() )
> +        {
> +            ret = -EAGAIN;
> +            break;
> +        }

And then this won't do what (I hope) you want on any continuation:
"start" doesn't get updated again in that case.

Jan


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