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SubjectRe: [bug report] resctrl high memory comsumption
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Hi Fenghua,

On 1/8/2020 12:23 PM, Fenghua Yu wrote:
> On Wed, Jan 08, 2020 at 09:07:41AM -0800, Shakeel Butt wrote:
>> Hi,
>>
>> Recently we had a bug in the system software writing the same pids to
>> the tasks file of resctrl group multiple times. The resctrl code
>> allocates "struct task_move_callback" for each such write and call
>> task_work_add() for that task to handle it on return to user-space
>> without checking if such request already exist for that particular
>> task. The issue arises for long sleeping tasks which has thousands for
>> such request queued to be handled. On our production, we notice
>> thousands of tasks having thousands of such requests and taking GiBs
>> of memory for "struct task_move_callback". I am not very familiar with
>> the code to judge if task_work_cancel() is the right approach or just
>> checking closid/rmid before doing task_work_add().
>>
>
> Thank you for reporting the issue, Shakeel!
>
> Could you please check if the following patch fixes the issue?
> From 3c23c39b6a44fdfbbbe0083d074dcc114d7d7f1c Mon Sep 17 00:00:00 2001
> From: Fenghua Yu <fenghua.yu@intel.com>
> Date: Wed, 8 Jan 2020 19:53:33 +0000
> Subject: [RFC PATCH] x86/resctrl: Fix redundant task movements
>
> Currently a task can be moved to a rdtgroup multiple times.
> But, this can cause multiple task works are added, waste memory
> and degrade performance.
>
> To fix the issue, only move the task to a rdtgroup when the task
> is not in the rdgroup. Don't try to move the task to the rdtgroup
> again when the task is already in the rdtgroup.
>
> Reported-by: Shakeel Butt <shakeelb@google.com>
> Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
> ---
> arch/x86/kernel/cpu/resctrl/rdtgroup.c | 11 +++++++++++
> 1 file changed, 11 insertions(+)
>
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index 2e3b06d6bbc6..75300c4a5969 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -546,6 +546,17 @@ static int __rdtgroup_move_task(struct task_struct *tsk,
> struct task_move_callback *callback;
> int ret;
>
> + /* If the task is already in rdtgrp, don't move the task. */
> + if ((rdtgrp->type == RDTCTRL_GROUP && tsk->closid == rdtgrp->closid &&
> + tsk->rmid == rdtgrp->mon.rmid) ||
> + (rdtgrp->type == RDTMON_GROUP &&
> + rdtgrp->mon.parent->closid == tsk->closid &&
> + tsk->rmid == rdtgrp->mon.rmid)) {
> + rdt_last_cmd_puts("Task is already in the rdgroup\n");
> +
> + return -EINVAL;
> + }
> +
> callback = kzalloc(sizeof(*callback), GFP_KERNEL);
> if (!callback)
> return -ENOMEM;
>

I think your fix would address this specific use case but a slightly
different use case will still encounter the problem of high memory
consumption. If for example, sleeping tasks are moved (many times)
between resource or monitoring groups then their task_works queue would
just keep growing. It seems that a call to task_work_cancel() before
adding a new work item should address all these cases?

Reinette

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