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SubjectRe: RCU latency regression in 2.6.16-rc1
Paul E. McKenney a écrit :
> On Sat, Jan 28, 2006 at 08:52:02PM +0100, Eric Dumazet wrote:
>> Dipankar Sarma a écrit :
>>> On Sat, Jan 28, 2006 at 01:51:23PM -0500, Lee Revell wrote:
>>>> On Sat, 2006-01-28 at 13:00 -0500, Lee Revell wrote:
>>>>> OK, now we are making progress.
>>>> I spoke too soon, it's not fixed:
>>>>
>>>> preemption latency trace v1.1.5 on 2.6.16-rc1
>>>> --------------------------------------------------------------------
>>>> latency: 4183 us, #3676/3676, CPU#0 | (M:rt VP:0, KP:0, SP:0 HP:0)
>>>> -----------------
>>>> evolutio-2877 0d.s. 97us : local_bh_enable (rt_run_flush)
>>>> evolutio-2877 0d.s. 98us : local_bh_enable (rt_run_flush)
>>>> evolutio-2877 0d.s. 99us : local_bh_enable (rt_run_flush)
>>>> evolutio-2877 0d.s. 100us : local_bh_enable (rt_run_flush)
>>>> evolutio-2877 0d.s. 101us : local_bh_enable (rt_run_flush)
>>>>
>>>> [ etc ]
>>>>
>>>> evolutio-2877 0d.s. 4079us : local_bh_enable (rt_run_flush)
>>>> evolutio-2877 0d.s. 4080us : local_bh_enable (rt_run_flush)
>>> I am not sure if I am interpreting the latency trace right,
>>> but it seems that there is a difference between the problem
>>> you were seeing earlier and now.
>>>
>>> In one of your earlier traces, I saw -
>>>
>>> <idle>-0 0d.s. 182us : dst_destroy (dst_rcu_free)
>>> <idle>-0 0d.s. 183us : ipv4_dst_destroy (dst_destroy)
>>>
>>> [ etc - zillions of dst_rcu_free()s deleted ]
>>>
>>> <idle>-0 0d.s. 13403us : dst_rcu_free (__rcu_process_callbacks)
>>> <idle>-0 0d.s. 13403us : dst_destroy (dst_rcu_free)
>>>
>>> This points to latency increase caused by lots and lots of
>>> RCU callbacks doing dst_rcu_free(). Do you still see those ?
>>>
>>> Your new trace shows that we are held up in in rt_run_flush().
>>> I guess we need to investigate why we spend so much time in rt_run_flush(),
>>> because of a big route table or the lock acquisitions.
>> Some machines have millions of entries in their route cache.
>>
>> I suspect we cannot queue all them (or only hash heads as your previous
>> patch) by RCU. Latencies and/or OOM can occur.
>>
>> What can be done is :
>>
>> in rt_run_flush(), allocate a new empty hash table, and exchange the hash
>> tables.
>>
>> Then wait a quiescent/grace RCU period (may be the exact term is not this
>> one, sorry, I'm not RCU expert)
>>
>> Then free all the entries from the old hash table (direclty of course, no
>> need for RCU grace period), and free the hash table.
>>
>> As the hash table can be huge, we might need allocate it at boot time, just
>> in case a flush is needed (it usually is :) ). If we choose dynamic
>> allocation and this allocation fails, then fallback to what is done today.
>
> Interesting approach!
>
> If I remember correctly, the point of all of this is to perturb the hash
> function periodically in order to avoid DoS attacks. It will likely
> be necessary to avoid a big performance hit during the transition.
> One way of doing this, given your two-table scheme, would be to:
>
> o Allocate both tables at boot time, as you suggest above.
>
> o Keep the following additional state:
>
> o Pointer to the table that is the current table.
>
> o First valid index (fvl) into the current table -- all
> indexes below the fvl correspond to hash buckets that
> have been transferred into the non-current table.
> In the normal case where the tables are not being
> switched, fvl==-1.
>
> (To make the RCU searches work without requiring
> tons of explicit memory barriers, there needs to
> be a separate fvl for each of the tables.)
>
> o Parameters defining the hash functions for the current
> table and for the non-current table.
>
> o When it is time to switch tables, start removing the entries
> in hash bucket #fvl of the current table. Optionally put them
> into the non-current table (or just let them be added as they
> are needed. Only remove a limited number of entries (or,
> alternatively, stop removing them after a limited amount of
> time).
>
> When the current hash bucket has been completely emptied,
> increment fvl, and, if we have not already hit the limit,
> continue on the new hash bucket.
>
> When fvl runs off the end of the table, you are done with
> the switch. Update the pointer to reference the other
> table. Important -- do -not- start another switch until
> a grace period has elapsed!!! Otherwise, you will end
> up fatally confusing slow readers.
>
> o When searching, if the hash function gives a value less
> than fvl, search the non-current table.
>
> If the hash function gives a value equal to fvl, search
> the current table, and, if not found, search the non-current
> table.
>
> If the hash function gives a value greater than fvl, search
> only the current table. (It may also be necessary to search
> the non-current table to allow for races with fvl update.)
>
> Does this seem reasonable?
>
> Thanx, Paul

Well, if as a bonus we are able to expand the size of the hash table, it could
be very very good : As of today, the boot time sizing of this hash table is
somewhat problematic.

If the size is expanded by a 2 factor (or a power of too), can your proposal
works ?

Eric
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