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Date: Thu, 13 Jun 2019 11:26:17 +0000
From: Vlad Buslov <vladbu@...lanox.com>
To: Jiri Pirko <jiri@...nulli.us>
CC: Vlad Buslov <vladbu@...lanox.com>,
"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
"pablo@...filter.org" <pablo@...filter.org>,
"xiyou.wangcong@...il.com" <xiyou.wangcong@...il.com>,
"jhs@...atatu.com" <jhs@...atatu.com>, mlxsw <mlxsw@...lanox.com>,
Alex Kushnarov <alexanderk@...lanox.com>,
"pabeni@...hat.com" <pabeni@...hat.com>
Subject: Re: tc tp creation performance degratation since kernel 5.1
On Thu 13 Jun 2019 at 14:11, Jiri Pirko <jiri@...nulli.us> wrote:
> Thu, Jun 13, 2019 at 12:09:32PM CEST, vladbu@...lanox.com wrote:
>>On Thu 13 Jun 2019 at 11:11, Jiri Pirko <jiri@...nulli.us> wrote:
>>> I made a mistake during measurements, sorry about that.
>>>
>>> This is the correct script:
>>> -----------------------------------------------------------------------
>>> #!/bin/bash
>>>
>>> dev=testdummy
>>> ip link add name $dev type dummy
>>> ip link set dev $dev up
>>> tc qdisc add dev $dev ingress
>>>
>>> tmp_file_name=$(date +"/tmp/tc_batch.%s.%N.tmp")
>>> pref_id=1
>>>
>>> while [ $pref_id -lt 20000 ]
>>> do
>>> echo "filter add dev $dev ingress proto ip pref $pref_id flower action drop" >> $tmp_file_name
>>> #echo "filter add dev $dev ingress proto ip pref $pref_id matchall action drop" >> $tmp_file_name
>>> ((pref_id++))
>>> done
>>>
>>> start=$(date +"%s")
>>> tc -b $tmp_file_name
>>> stop=$(date +"%s")
>>> echo "Insertion duration: $(($stop - $start)) sec"
>>> rm -f $tmp_file_name
>>>
>>> ip link del dev $dev
>>> -----------------------------------------------------------------------
>>>
>>> Note the commented out matchall. I don't see the regression with
>>> matchall. However, I see that with flower:
>>> kernel 5.1
>>> Insertion duration: 4 sec
>>> kernel 5.2
>>> Insertion duration: 163 sec
>>>
>>> I don't see any significant difference in perf:
>>> kernel 5.1
>>> 77.24% tc [kernel.vmlinux] [k] tcf_chain_tp_find
>>> 1.67% tc [kernel.vmlinux] [k] mutex_spin_on_owner
>>> 1.44% tc [kernel.vmlinux] [k] _raw_spin_unlock_irqrestore
>>> 0.93% tc [kernel.vmlinux] [k] idr_get_free
>>> 0.79% tc_pref_scale_o [kernel.vmlinux] [k] do_syscall_64
>>> 0.69% tc [kernel.vmlinux] [k] finish_task_switch
>>> 0.53% tc libc-2.28.so [.] __memset_sse2_unaligned_erms
>>> 0.49% tc [kernel.vmlinux] [k] __memset
>>> 0.36% tc_pref_scale_o libc-2.28.so [.] malloc
>>> 0.30% tc_pref_scale_o libc-2.28.so [.] _int_free
>>> 0.24% tc [kernel.vmlinux] [k] __memcpy
>>> 0.23% tc [cls_flower] [k] fl_change
>>> 0.23% tc [kernel.vmlinux] [k] __nla_validate_parse
>>> 0.22% tc [kernel.vmlinux] [k] __slab_alloc
>>>
>>>
>>> 75.57% tc [kernel.kallsyms] [k] tcf_chain_tp_find
>>> 2.70% tc [kernel.kallsyms] [k] _raw_spin_unlock_irqrestore
>>> 1.13% tc_pref_scale_o [kernel.kallsyms] [k] do_syscall_64
>>> 0.87% tc libc-2.28.so [.] __memset_sse2_unaligned_erms
>>> 0.86% ip [kernel.kallsyms] [k] finish_task_switch
>>> 0.67% tc [kernel.kallsyms] [k] memset
>>> 0.63% tc [kernel.kallsyms] [k] mutex_spin_on_owner
>>> 0.52% tc_pref_scale_o libc-2.28.so [.] malloc
>>> 0.48% tc [kernel.kallsyms] [k] idr_get_free
>>> 0.46% tc [kernel.kallsyms] [k] fl_change
>>> 0.42% tc_pref_scale_o libc-2.28.so [.] _int_free
>>> 0.35% tc_pref_scale_o libc-2.28.so [.] __GI___strlen_sse2
>>> 0.35% tc_pref_scale_o libc-2.28.so [.] __mbrtowc
>>> 0.34% tc_pref_scale_o libc-2.28.so [.] __fcntl64_nocancel_adjusted
>>>
>>> Any ideas?
>>
>>Thanks for providing reproduction script!
>>
>>I've investigate the problem and found the root cause. First of all I
>>noticed that CPU utilization during problematic tc run is quite low
>>(<10%), so I decided to investigate why tc sleeps so much. I've used bcc
>>and obtained following off-CPU trace (uninteresting traces are omitted
>>for brevity):
>>
>>~$ sudo /usr/share/bcc/tools/offcputime -K -p `pgrep -nx tc`
>>Tracing off-CPU time (us) of PID 2069 by kernel stack... Hit Ctrl-C to end.
>>...
>> finish_task_switch
>> __sched_text_start
>> schedule
>> schedule_timeout
>> wait_for_completion
>> __wait_rcu_gp
>> synchronize_rcu
>> fl_change
>> tc_new_tfilter
>> rtnetlink_rcv_msg
>> netlink_rcv_skb
>> netlink_unicast
>> netlink_sendmsg
>> sock_sendmsg
>> ___sys_sendmsg
>> __sys_sendmsg
>> do_syscall_64
>> entry_SYSCALL_64_after_hwframe
>> - tc (2069)
>> 142284953
>>
>>As you can see 142 seconds are spent sleeping in synchronize_rcu(). The
>>code is in fl_create_new_mask() function:
>>
>> err = rhashtable_replace_fast(&head->ht, &mask->ht_node,
>> &newmask->ht_node, mask_ht_params);
>> if (err)
>> goto errout_destroy;
>>
>> /* Wait until any potential concurrent users of mask are finished */
>> synchronize_rcu();
>>
>>The justification for this is described in comment in
>>fl_check_assign_mask() (user of fl_create_new_mask()):
>>
>> /* Insert mask as temporary node to prevent concurrent creation of mask
>> * with same key. Any concurrent lookups with same key will return
>> * -EAGAIN because mask's refcnt is zero. It is safe to insert
>> * stack-allocated 'mask' to masks hash table because we call
>> * synchronize_rcu() before returning from this function (either in case
>> * of error or after replacing it with heap-allocated mask in
>> * fl_create_new_mask()).
>> */
>> fnew->mask = rhashtable_lookup_get_insert_fast(&head->ht,
>> &mask->ht_node,
>> mask_ht_params);
>>
>>The offending commit is part of my series that implements unlocked
>>flower: 195c234d15c9 ("net: sched: flower: handle concurrent mask
>>insertion")
>>
>>The justification presented in it is no longer relevant since Ivan
>>Vecera changed mask to be dynamically allocated in commit 2cddd2014782
>>("net/sched: cls_flower: allocate mask dynamically in fl_change()").
>>With this we can just change fl_change() to deallocate temporary mask
>>with rcu grace period and remove offending syncrhonize_rcu() call.
>>
>>Any other suggestions?
>
> So basically you just change synchronize_rcu() to kfree_rcu(mask),
> correct?
Not really. I remove synchronize_rcu() and change all kfree(mask) in
fl_change() to tcf_queue_work(&mask->rwork, fl_mask_free_work) which
uses queue_rcu_work() internally. This would allow us to deallocate
fl_flow_mask in same manner on all code paths and doesn't require any
extensions in fl_flow_mask struct (kfree_rcu would require extending it
with rcu_head).
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