Re: [Xen-devel] [PATCH 2/2] grant_table: convert grant table rwlock to percpu rwlock

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 18/11/15 20:02, Konrad Rzeszutek Wilk wrote:
> On Tue, Nov 17, 2015 at 05:30:59PM +0000, Andrew Cooper wrote:
>> On 17/11/15 17:04, Jan Beulich wrote:
>>>>>> On 03.11.15 at 18:58, <malcolm.crossley@xxxxxxxxxx> wrote:
>>>> --- a/xen/common/grant_table.c
>>>> +++ b/xen/common/grant_table.c
>>>> @@ -178,6 +178,10 @@ struct active_grant_entry {
>>>>  #define _active_entry(t, e) \
>>>>      ((t)->active[(e)/ACGNT_PER_PAGE][(e)%ACGNT_PER_PAGE])
>>>>  
>>>> +bool_t grant_rwlock_barrier;
>>>> +
>>>> +DEFINE_PER_CPU(rwlock_t *, grant_rwlock);
>>> Shouldn't these be per grant table? And wouldn't doing so eliminate
>>> the main limitation of the per-CPU rwlocks?
>> The grant rwlock is per grant table.
>>
>> The entire point of this series is to reduce the cmpxchg storm which
>> happens when many pcpus attempt to grap the same domains grant read lock.
>>
>> As identified in the commit message, reducing the cmpxchg pressure on
>> the cache coherency fabric increases intra-vm network through from
>> 10Gbps to 50Gbps when running iperf between two 16-vcpu guests.
>>
>> Or in other words, 80% of cpu time is wasted with waiting on an atomic
>> read/modify/write operation against a remote hot cache line.
>>
> Why not use MCE locks then (in Linux the implemention is known
> as qspinlock). Plus they have added extra code to protect against
> recursion (via four levels). See Linux commit
> a33fda35e3a7655fb7df756ed67822afb5ed5e8d
> locking/qspinlock: Introduce a simple generic 4-byte queued spinlock)

The bottlekneck here is the act of taking the read lock, even when it
will be successfully acquired.

There are several good reasons to move to MCS locks (not MCE locks,
which sound somewhat disastrous), but they will not help in the
slightest with this issue.

The optimisation here is to avoid taking the read lock all-together in
the (overwhelmingly) common case.

~Andrew

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