linux-kernel - Re: [PATCH v2 3/3] IB/core: Obtain subnet

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Date:   Thu, 3 Jun 2021 09:29:32 +0000
From:   Haakon Bugge <haakon.bugge@...cle.com>
To:     Leon Romanovsky <leon@...nel.org>
CC:     Anand Khoje <anand.a.khoje@...cle.com>,
        OFED mailing list <linux-rdma@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dledford@...hat.com" <dledford@...hat.com>,
        "jgg@...pe.ca" <jgg@...pe.ca>
Subject: Re: [PATCH v2 3/3] IB/core: Obtain subnet_prefix from cache in IB
 devices



> On 3 Jun 2021, at 11:07, Leon Romanovsky <leon@...nel.org> wrote:
> 
> On Thu, Jun 03, 2021 at 12:20:24PM +0530, Anand Khoje wrote:
>> ib_query_port() calls device->ops.query_port() to get the port
>> attributes. The method of querying is device driver specific.
>> The same function calls device->ops.query_gid() to get the GID and
>> extract the subnet_prefix (gid_prefix).
>> 
>> The GID and subnet_prefix are stored in a cache. But they do not get
>> read from the cache if the device is an Infiniband device. The
>> following change takes advantage of the cached subnet_prefix.
>> Testing with RDBMS has shown a significant improvement in performance
>> with this change.
>> 
>> The function ib_cache_is_initialised() is introduced because
>> ib_query_port() gets called early in the stage when the cache is not
>> built while reading port immutable property.
>> 
>> In that case, the default GID still gets read from HCA for IB link-
>> layer devices.
>> 
>> Fixes: fad61ad ("IB/core: Add subnet prefix to port info")
>> Signed-off-by: Anand Khoje <anand.a.khoje@...cle.com>
>> Signed-off-by: Haakon Bugge <haakon.bugge@...cle.com>
>> ---
>> drivers/infiniband/core/cache.c  | 7 ++++++-
>> drivers/infiniband/core/device.c | 9 +++++++++
>> include/rdma/ib_cache.h          | 6 ++++++
>> include/rdma/ib_verbs.h          | 6 ++++++
>> 4 files changed, 27 insertions(+), 1 deletion(-)
> 
> Can you please help me to understand how cache is updated?
> 
> There are a lot of calls to ib_query_port() and I wonder how callers can
> get new GID after it was changed in already initialized cache.

The cache is initialized when it is created, just before the bit IB_PORT_CACHE_INITIALIZED is set in flags.

After commit d58c23c92548 ("IB/core: Only update PKEY and GID caches on respective events"), the GID portion of the cache is updated when a IB_EVENT_GID_CHANGE event is received.

Before said commit, it was updated on any event.


Thxs, Håkon