[ovs-dev] [PATCHv6 11/14] datapath: Add support for unique flow identifiers.

[Joe Stringer]

On 1 October 2014 11:59, Joe Stringer <joestringer at nicira.com> wrote:

>
>
> On 1 October 2014 11:55, Pravin Shelar <pshelar at nicira.com> wrote:
>
>> On Tue, Sep 30, 2014 at 3:15 PM, Joe Stringer <joestringer at nicira.com>
>> wrote:
>> >
>> >
>> > On 1 October 2014 06:56, Pravin Shelar <pshelar at nicira.com> wrote:
>> >>
>> >> On Mon, Sep 29, 2014 at 3:59 PM, Joe Stringer <joestringer at nicira.com>
>> >> wrote:
>> >> >
>> >> >
>> >> > On 30 September 2014 10:10, Ben Pfaff <blp at nicira.com> wrote:
>> >> >>
>> >> >> On Fri, Sep 26, 2014 at 09:28:15PM +1200, Joe Stringer wrote:
>> >> >> > If a datapath is created with the flag OVS_DP_F_INDEX_BY_UID,
>> then an
>> >> >> > additional table_instance is added to the flow_table, which is
>> >> >> > indexed
>> >> >> > by unique identifiers ("UID"). Userspace implementations can
>> specify
>> >> >> > a
>> >> >> > UID of up to 128 bits along with a flow operation as shorthand for
>> >> >> > the
>> >> >> > key. This allows revalidation performance improvements of up to
>> 50%.
>> >> >> >
>> >> >> > If a datapath is created using OVS_DP_F_INDEX_BY_UID and a UID is
>> not
>> >> >> > specified at flow setup time, then that operation will fail. If
>> >> >> > OVS_UID_F_* flags are specified for an operation, then they will
>> >> >> > modify
>> >> >> > what is returned through the operation. For instance,
>> >> >> > OVS_UID_F_SKIP_KEY
>> >> >> > allows the datapath to skip returning the key (eg, during dump to
>> >> >> > reduce
>> >> >> > memory copy).
>> >> >> >
>> >> >> > Signed-off-by: Joe Stringer <joestringer at nicira.com>
>> >> >> > ---
>> >> >> > v6: Fix documentation for supporting UIDs between 32-128 bits.
>> >> >> >     Minor style fixes.
>> >> >> >     Rebase.
>> >> >> > v5: No change.
>> >> >> > v4: Fix memory leaks.
>> >> >> >     Log when triggering the older userspace issue above.
>> >> >> > v3: Initial post.
>> >> >>
>> >> >> This review is from an ABI standpoint only; it's not a review of the
>> >> >> kernel code itself.
>> >> >>
>> >> >> OVS_UID_ATTR_ID is marked as 32-128 bits long.  For the "userdata"
>> >> >> attribute of the userspace action, we originally had it fixed at 64
>> >> >> bits, then later we decided that it was more flexible to allow it to
>> >> >> be any size.  Is there an advantage to fixing it within this range?
>> >> >
>> >> >
>> >> > I'm not sure there's any advantage, that's just the way it's written
>> >> > right
>> >> > now. Perhaps with a bit of tweaking, we could get rid of
>> MAX_UID_BUFSIZE
>> >> > and
>> >> > have no restrictions on the size of this.
>> >> >
>> >> >>
>> >> >>
>> >> >> I'm a little surprised that OVS_DP_F_INDEX_BY_UID is necessary.  In
>> >> >> the past we've only added flags for features that otherwise
>> required a
>> >> >> backward-incompatible change to the datapath interface.  Is adding a
>> >> >> UID such a change?
>> >> >
>> >> >
>> >> > Pravin had some preferences on this during the original drafting,
>> but I
>> >> > can't find a direct requirement for this. The alternative means that
>> >> > flows
>> >> > might not be present in both of the hastables (indexed by UID vs.
>> exact
>> >> > flow_key), although they would always need to be in the exact
>> flow_key
>> >> > table. Might be worth bouncing this off Pravin to see if I'm on the
>> mark
>> >> > with how I've used it here.
>> >>
>> >> I looked into the patch and I think we can get rid of
>> >> OVS_DP_F_INDEX_BY_UID.
>> >> On flow insert we can use flow-id provided by userspace, if it is not
>> >> passed we can generate in kernel and use it to insert it in the UID
>> >> hash table. sw_flow can have a flag set for kernel generated flow-uid,
>> >> this can be used along with OVS_UID_F_SKIP_KEY in flow dump operation
>> >> to return key to userspace or not. On flow dump can always iterate UID
>> >> hash table where flow iteration should be relatively stable.
>> >
>> >
>> > OK, so in this case generally when userspace doesn't support UIDs, the
>> > datapath will generate them mainly to keep the flow-id and flow-key
>> > hashtables in sync. (Currently, we just disable the flow-id hashtable).
>> >
>> > Currently, OVS_UID_F_SKIP_KEY is a request flag that means "omit the
>> flow
>> > key". Are you suggesting that it should only omit flow keys for flows
>> which
>> > have a userspace-specified flow-id?
>> >
>> I used wrong wording, What I mean is keep OVS_UID_F_SKIP_KEY semantic
>> as it is and do not return kernel generated uids to userspace. Since I
>> do not see any use of kernel uids to userspace.
>>
>
> OK, this sounds quite reasonable.
>

Somehow it slipped my mind earlier, if the OVS_UID_F_SKIP_KEY flag is set,
and a flow doesn't have a UID, then does flow dump skip the flow entirely,
or return the key for it? I'm leaning towards the latter. Already the
OVS_UID_F_SKIP_KEY semantics are a request that might be ignored (eg, newer
userspace with older datapath), and it seems better to include all of the
flows to catch corner cases like this.