From mboxrd@z Thu Jan  1 00:00:00 1970
From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Subject: Re: [PATCH v12 for-xen-4.5 11/20] x86/VPMU: Interface
 for setting PMU mode and flags
Date: Mon, 29 Sep 2014 09:56:50 -0400
Message-ID: <54296522.7060001@oracle.com>
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To: Dietmar Hahn <dietmar.hahn@ts.fujitsu.com>, xen-devel@lists.xen.org
Cc: kevin.tian@intel.com, keir@xen.org, jbeulich@suse.com, jun.nakajima@intel.com, andrew.cooper3@citrix.com, tim@xen.org, Aravind.Gopalakrishnan@amd.com, suravee.suthikulpanit@amd.com, dgdegra@tycho.nsa.gov
List-Id: xen-devel@lists.xenproject.org

On 09/29/2014 09:25 AM, Dietmar Hahn wrote:
> +static int vpmu_force_context_switch(void)
> +{
> +    unsigned i, j, allbutself_num, mycpu;
> +    static s_time_t start, now;
> +    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);
> +    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);
> Only a question of understanding.
> Is there a special reason not to use a single memory allocation
> except for memory fragmentation on systems with a large number of cpus?
>
>       struct tasklet *sync_task;
>       sync_task = xmalloc(sizeof(struct tasklet) * allbutself_num);


Exactly because of fragmentation -- this will avoid asking for more than 
a page during runtime. I, in fact, originally had it allocated as a 
single chunk, just as you suggested above, but Jan asked this to be 
split into smaller, sub-page pieces.

-boris