From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Vrabel <david.vrabel@citrix.com>
Subject: Re: [PATCHv6 3/5] gnttab: split grant table lock into
 table and maptrack locks
Date: Wed, 29 Apr 2015 11:53:50 +0100
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To: Jan Beulich <JBeulich@suse.com>, David Vrabel <david.vrabel@citrix.com>
Cc: Keir Fraser <keir@xen.org>, Ian Campbell <ian.campbell@citrix.com>, Christoph Egger <chegger@amazon.de>, Tim Deegan <tim@xen.org>, Matt Wilson <msw@amazon.com>, xen-devel@lists.xenproject.org
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On 23/04/15 16:04, Jan Beulich wrote:
>
>> --- a/xen/include/xen/grant_table.h
>> +++ b/xen/include/xen/grant_table.h
>> @@ -82,7 +82,12 @@ struct grant_table {
>>      struct grant_mapping **maptrack;
>>      unsigned int          maptrack_head;
>>      unsigned int          maptrack_limit;
>> -    /* Lock protecting updates to active and shared grant tables. */
>> +    /* Lock protecting the maptrack page list, head, and limit */
>> +    spinlock_t            maptrack_lock;
>> +    /* 
>> +     * Lock protecting updates to grant table state (version, active
>> +     * entry list, etc.)
>> +     */
>>      spinlock_t            lock;
> 
> If the patch still was to be applied, these two locks should be put
> on separate cache lines, to avoid unnecessary bouncing.

I was shuffling the fields around to make this happen and I ended up
shrinking the structure to fit it in a single cache line instead.

Shall I add some explicit padding to get the alignment you want?  Note
that with the per-CPU maptrack free lists the maptrack lock isn't
heavily contended.

David