netdev - Re: [PATCH 03/15] devlink: add operation to take an immediate snapshot

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Date:   Mon, 3 Feb 2020 22:30:57 +0100
From:   Jiri Pirko <jiri@...nulli.us>
To:     Jacob Keller <jacob.e.keller@...el.com>
Cc:     netdev@...r.kernel.org, valex@...lanox.com, linyunsheng@...wei.com,
        lihong.yang@...el.com
Subject: Re: [PATCH 03/15] devlink: add operation to take an immediate
 snapshot

Mon, Feb 03, 2020 at 08:32:36PM CET, jacob.e.keller@...el.com wrote:
>On 2/3/2020 3:50 AM, Jiri Pirko wrote:
>> Thu, Jan 30, 2020 at 11:58:58PM CET, jacob.e.keller@...el.com wrote:
>>> Add a new devlink command, DEVLINK_CMD_REGION_TAKE_SNAPSHOT. This
>>> command is intended to enable userspace to request an immediate snapshot
>>> of a region.
>>>
>>> Regions can enable support for requestable snapshots by implementing the
>>> snapshot callback function in the region's devlink_region_ops structure.
>>>
>>> Implementations of this function callback should capture an immediate
>>> copy of the data and return it and its destructor in the function
>>> parameters. The core devlink code will generate a snapshot ID and create
>>> the new snapshot while holding the devlink instance lock.
>>>
>>> Signed-off-by: Jacob Keller <jacob.e.keller@...el.com>
>>> ---
>>> .../networking/devlink/devlink-region.rst     |  9 +++-
>>> include/net/devlink.h                         |  7 +++
>>> include/uapi/linux/devlink.h                  |  2 +
>>> net/core/devlink.c                            | 46 +++++++++++++++++++
>>> 4 files changed, 62 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/Documentation/networking/devlink/devlink-region.rst b/Documentation/networking/devlink/devlink-region.rst
>>> index 1a7683e7acb2..262249e6c3fc 100644
>>> --- a/Documentation/networking/devlink/devlink-region.rst
>>> +++ b/Documentation/networking/devlink/devlink-region.rst
>>> @@ -20,6 +20,11 @@ address regions that are otherwise inaccessible to the user.
>>> Regions may also be used to provide an additional way to debug complex error
>>> states, but see also :doc:`devlink-health`
>>>
>>> +Regions may optionally support capturing a snapshot on demand via the
>>> +``DEVLINK_CMD_REGION_TAKE_SNAPSHOT`` netlink message. A driver wishing to
>>> +allow requested snapshots must implement the ``.snapshot`` callback for the
>>> +region in its ``devlink_region_ops`` structure.
>>> +
>>> example usage
>>> -------------
>>>
>>> @@ -40,8 +45,8 @@ example usage
>>>     # Delete a snapshot using:
>>>     $ devlink region del pci/0000:00:05.0/cr-space snapshot 1
>>>
>>> -    # Trigger (request) a snapshot be taken:
>>> -    $ devlink region trigger pci/0000:00:05.0/cr-space
>>> +    # Request an immediate snapshot, if supported by the region
>>> +    $ devlink region snapshot pci/0000:00:05.0/cr-space
>> 
>> 
>> Hmm, the shapshot is now removed by user calling:
>> 
>> $ devlink region del DEV/REGION snapshot SNAPSHOT_ID
>> That is using DEVLINK_CMD_REGION_DEL netlink command calling
>> devlink_nl_cmd_region_del()
>> 
>> I think the creation should be symmetric. Something like:
>> $ devlink region add DEV/REGION snapshot SNAPSHOT_ID
>> SNAPSHOT_ID is either exact number or "any" if user does not care.
>> The benefit of using user-passed ID value is that you can use this
>> easily in scripts.
>> 
>> The existing unused netlink command DEVLINK_CMD_REGION_NEW would be used
>> for this.
>> 
>
>So I have some concern trying to allow picking the snapshot id. I agree
>it is useful, but want to make sure we pick the best design for how to
>handle things.
>
>Currently regions support taking a snapshot across multiple regions with
>the same ID. this means that the region id value is stored per devlink
>instead of per region.
>
>If users can pick IDs, they can and probably will become sparse. This
>means that we now need to be able to handle this.
>
>If a user picks an ID, we want to ensure that the global region id
>number is incremented properly so that we skip the used IDs, otherwise
>those could accidentally collide.
>
>The simplest solution is to just force the global ID to be 1 larger at a
>minimum every time the userspace calls us with an ID.
>
>But now what happens if a user requests a really large ID (U32_MAX - 1)?
>and then we now overflow our region ID.
>
>This was previously a rare occurrence, but has now become possibly common.
>
>We could require/force the user to pick IDs within a limited range, and
>have the automatic regions come from another range..
>
>We could enhance ID selection to just pick "lowest number unused by any
>region". This would allow re-using ID numbers after they've been
>deleted.. I think this approach is the most robust but does require a
>bit of extra computation.

Check out "ida"

>
>Thanks,
>Jake