netdev - Re: [PATCH bpf-next v2 1/2] bpf: avoid iterating duplicated files for task

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Message-ID: <31e47eed-0ac0-b221-16ba-5284c1ec6386@fb.com>
Date:   Tue, 1 Sep 2020 10:38:13 -0700
From:   Yonghong Song <yhs@...com>
To:     Josef Bacik <josef@...icpanda.com>, <bpf@...r.kernel.org>,
        <netdev@...r.kernel.org>
CC:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, <kernel-team@...com>,
        Andrii Nakryiko <andriin@...com>
Subject: Re: [PATCH bpf-next v2 1/2] bpf: avoid iterating duplicated files for
 task_file iterator



On 9/1/20 10:18 AM, Josef Bacik wrote:
> On 8/28/20 1:38 AM, Yonghong Song wrote:
>> Currently, task_file iterator iterates all files from all tasks.
>> This may potentially visit a lot of duplicated files if there are
>> many tasks sharing the same files, e.g., typical pthreads
>> where these pthreads and the main thread are sharing the same files.
>>
>> This patch changed task_file iterator to skip a particular task
>> if that task shares the same files as its group_leader (the task
>> having the same tgid and also task->tgid == task->pid).
>> This will preserve the same result, visiting all files from all
>> tasks, and will reduce runtime cost significantl, e.g., if there are
>> a lot of pthreads and the process has a lot of open files.
>>
>> Suggested-by: Andrii Nakryiko <andriin@...com>
>> Signed-off-by: Yonghong Song <yhs@...com>
>> ---
>>   kernel/bpf/task_iter.c | 14 +++++++++-----
>>   1 file changed, 9 insertions(+), 5 deletions(-)
>>
>> It would be good if somebody familar with sched code can help check
>> whether I missed anything or not (e.g., locks, etc.)
>> for the code change
>>    task->files == task->group_leader->files
>>
>> Note the change in this patch might have conflicts with
>> e60572b8d4c3 ("bpf: Avoid visit same object multiple times")
>> which is merged into bpf/net sometimes back.
>>
>> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
>> index 232df29793e9..0c5c96bb6964 100644
>> --- a/kernel/bpf/task_iter.c
>> +++ b/kernel/bpf/task_iter.c
>> @@ -22,7 +22,8 @@ struct bpf_iter_seq_task_info {
>>   };
>>   static struct task_struct *task_seq_get_next(struct pid_namespace *ns,
>> -                         u32 *tid)
>> +                         u32 *tid,
>> +                         bool skip_if_dup_files)
>>   {
>>       struct task_struct *task = NULL;
>>       struct pid *pid;
>> @@ -32,7 +33,10 @@ static struct task_struct *task_seq_get_next(struct 
>> pid_namespace *ns,
>>       pid = idr_get_next(&ns->idr, tid);
>>       if (pid) {
>>           task = get_pid_task(pid, PIDTYPE_PID);
>> -        if (!task) {
>> +        if (!task ||
>> +            (skip_if_dup_files &&
>> +             task->tgid != task->pid &&
>> +             task->files == task->group_leader->files)) {
>>               ++*tid;
>>               goto retry;
> 
> Sorry I only checked the task->files and task->group_leader thing, I 
> forgot to actually pay attention to what the patch itself was doing.
> 
> This will leak task structs, you need something like
> 
> if (!task) {
>      ++*tid;
>      goto retry;
> }
> if (skip_if_dup_files && etc) {
>      ++*tid;
>      put_task_struct(task);
>      goto retry;
> }
> 
> otherwise you'll leak tasks.  Thanks,

Or, yes, good catch. Thanks! I too focused on task->files == 
task->group_lead->files and did not think deep on leaking
tasks.

Will send v2.

> 
> Josef