linux-kernel - Re: [PATCH v5 bpf-next 1/6] bpf: enable task local storage for tracing programs

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Message-ID: <20210225192859.pfvkox7dgz236b3w@kafai-mbp.dhcp.thefacebook.com>
Date:   Thu, 25 Feb 2021 11:28:59 -0800
From:   Martin KaFai Lau <kafai@...com>
To:     Song Liu <songliubraving@...com>
CC:     <bpf@...r.kernel.org>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <ast@...nel.org>,
        <daniel@...earbox.net>, <kernel-team@...com>,
        <peterz@...radead.org>, kernel test robot <lkp@...el.com>,
        KP Singh <kpsingh@...nel.org>
Subject: Re: [PATCH v5 bpf-next 1/6] bpf: enable task local storage for
 tracing programs

On Tue, Feb 23, 2021 at 02:28:40PM -0800, Song Liu wrote:
> To access per-task data, BPF programs usually creates a hash table with
> pid as the key. This is not ideal because:
>  1. The user need to estimate the proper size of the hash table, which may
>     be inaccurate;
>  2. Big hash tables are slow;
>  3. To clean up the data properly during task terminations, the user need
>     to write extra logic.
> 
> Task local storage overcomes these issues and offers a better option for
> these per-task data. Task local storage is only available to BPF_LSM. Now
> enable it for tracing programs.
> 
> Unlike LSM programs, tracing programs can be called in IRQ contexts.
> Helpers that access task local storage are updated to use
> raw_spin_lock_irqsave() instead of raw_spin_lock_bh().
> 
> Tracing programs can attach to functions on the task free path, e.g.
> exit_creds(). To avoid allocating task local storage after
> bpf_task_storage_free(). bpf_task_storage_get() is updated to not allocate
> new storage when the task is not refcounted (task->usage == 0).
Acked-by: Martin KaFai Lau <kafai@...com>