Re: [PATCH] opensm/osm_state_mgr.c: force heavy sweep when fabric consists of single switch

Line Holen
2009-11-04T17:42:52Z

On 11/ 4/09 04:54 PM, Yevgeny Kliteynik wrote:
> Line Holen wrote:
>> On 11/ 4/09 10:47 AM, Yevgeny Kliteynik wrote:
>>> Sasha Khapyorsky wrote:
>>>> On 12:26 Tue 03 Nov , Yevgeny Kliteynik wrote:
>>>>> Always do heavy sweep when there is only one node in the
>>>>> fabric, and this node is a switch, and SM runs on top of it -
>>>>> there may be a race when OSM starts running before the
>>>>> external ports are ports are up, or if they went through
>>>>> reset while SM was starting.
>>>>> In this race switch brings up the ports and turns on the
>>>>> PSC bit, but OSM might get PortInfo before SwitchInfo, and it
>>>>> might see all ports as down, but PSC bit on. If that happens,
>>>>> OSM turns off PSC bit, and it will never see external ports
>>>>> again - it won't perform any heavy sweep, only light sweep
>>>> Could such race happen when there are more than one node in a fabric?
>>> I think that my description of the race was misleading.
>>> The race can happen on *any* fabric when SM runs on switch.
>>> But when it does happen, SM thinks that the whole subnet
>>> is just one switch - that's what it managed to discover.
>>> I've actually seen it happening.
>>> So the patch fixes this particular case.
>>>
>>> So the next question that you would probably ask is can
>>> this race happen on some *other* switch and not the one
>>> SM is running on?
>>>
>>> Well, I don't know. I have a hunch that it can't, but I
>>> couldn't prove it to myself yet.
>>>
>>> The race on the managed switch is a special case because
>>> SM always sees port 0, and always gets responses to its
>>> SMP queries. On any other switch, if the ports were reset,
>>> SM won't get any response until the ports are up again.
>>>
>>> Perhaps there might be a case where SM got some port as down,
>>> and by the time SM got SwitchInfo with PSC bit the port
>>> was already up, so SM won't start discovery beyond this
>>> port. But this race would be fixed on the next heavy sweep,
>>> when SM will discover this port that it missed the previous
>>> time, whereas race on managed switch is fatal - SM won't
>>> ever do any heavy sweep.
>>>
>>> -- Yevgeny
>>
>> At least for the 3.2 branch there is a general race regardless of
>> where the SM is running. I haven't checked the current master, but
>> I cannot recall seeing any patches related to this so I assume
>> the race is still there.
>>
>> There is a window between SM discovering a switch and clearing PSC
>> for the same switch. The SM will not detect a state change on the
>> switch ports during this time.
>
> If the port changes state during that period, the switch issues
> new trap 128, which (I think) should cause SM to re-discover the
> fabric once this discovery cycle is over. Is this correct?
>

I think the switch shall send a trap whenever it sets the PSC bit.
Once set I believe it will not send another trap until it is reset.
Or do I misinterpret the spec ?

> Or perhaps the more serious problem happens when SM LID is not
> configured yet on the switch, hence the trap is not going to the
> right place?
>
>> I have a patch for the 3.2 branch that I can merge into master.
>
> Sure, that would be nice :)
>
> -- Yevgeny
>
>
>> Line
>>
>>>> Sasha
>>>>
>>>>> Signed-off-by: Yevgeny Kliteynik <kliteyn-LDSdmyG8hGV8YrgS2mwiifqBs+8SCbDb@public.gmane.org>
>>>>> ---
>>>>> opensm/opensm/osm_state_mgr.c | 15 ++++++++++-----
>>>>> 1 files changed, 10 insertions(+), 5 deletions(-)
>>>>>
>>>>> diff --git a/opensm/opensm/osm_state_mgr.c
>>>>> b/opensm/opensm/osm_state_mgr.c
>>>>> index 4303d6e..537c855 100644
>>>>> --- a/opensm/opensm/osm_state_mgr.c
>>>>> +++ b/opensm/opensm/osm_state_mgr.c
>>>>> @@ -1062,13 +1062,18 @@ static void do_sweep(osm_sm_t * sm)
>>>>> * Otherwise, this is probably our first discovery pass
>>>>> * or we are connected in loopback. In both cases do a
>>>>> * heavy sweep.
>>>>> - * Note: If we are connected in loopback we want a heavy
>>>>> - * sweep, since we will not be getting any traps if there is
>>>>> - * a lost connection.
>>>>> + * Note the following:
>>>>> + * 1. If we are connected in loopback we want a heavy sweep,
>>>>> since we
>>>>> + * will not be getting any traps if there is a lost
>>>>> connection.
>>>>> + * 2. If we are in DISCOVERING state - this means it is either in
>>>>> + * initializing or wake up from STANDBY - run the heavy sweep.
>>>>> + * 3. If there is only one node in the fabric, and this node is a
>>>>> + * switch, and OSM runs on top of it, there might be a race
>>>>> when
>>>>> + * OSM starts running before the external ports are up -
>>>>> run the
>>>>> + * heavy sweep.
>>>>> */
>>>>> - /* if we are in DISCOVERING state - this means it is either in
>>>>> - * initializing or wake up from STANDBY - run the heavy sweep */
>>>>> if (cl_qmap_count(&sm->p_subn->sw_guid_tbl)
>>>>> + && cl_qmap_count(&sm->p_subn->node_guid_tbl) != 1
>>>>> && sm->p_subn->sm_state != IB_SMINFO_STATE_DISCOVERING
>>>>> && sm->p_subn->opt.force_heavy_sweep == FALSE
>>>>> && sm->p_subn->force_heavy_sweep == FALSE
>>>>> --
>>>>> 1.5.1.4
>>>>>
>>>>> --
>>>>> To unsubscribe from this list: send the line "unsubscribe
>>>>> linux-rdma" in
>>>>> the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
>>>>> More majordomo info at http://vger.kernel.org/majordomo-info.html
>>>>>
>>> --
>>> To unsubscribe from this list: send the line "unsubscribe linux-rdma" in
>>> the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
>>> More majordomo info at http://vger.kernel.org/majordomo-info.html