Re: [PATCH v2 1/1] psi: remove 500ms min window size limitation for triggers

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Mar 2, 2023 at 5:13 PM Suren Baghdasaryan <surenb@google.com> wrote:
>
> Current 500ms min window size for psi triggers limits polling interval
> to 50ms to prevent polling threads from using too much cpu bandwidth by
> polling too frequently. However the number of cgroups with triggers is
> unlimited, so this protection can be defeated by creating multiple
> cgroups with psi triggers (triggers in each cgroup are served by a single
> "psimon" kernel thread).
> Instead of limiting min polling period, which also limits the latency of
> psi events, it's better to limit psi trigger creation to authorized users
> only, like we do for system-wide psi triggers (/proc/pressure/* files can
> be written only by processes with CAP_SYS_RESOURCE capability). This also
> makes access rules for cgroup psi files consistent with system-wide ones.
> Add a CAP_SYS_RESOURCE capability check for cgroup psi file writers and
> remove the psi window min size limitation.
>
> Suggested-by: Sudarshan Rajagopalan <quic_sudaraja@quicinc.com>
> Link: https://lore.kernel.org/all/cover.1676067791.git.quic_sudaraja@quicinc.com/
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> Acked-by: Michal Hocko <mhocko@suse.com>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Forgot to change the --to field from Tejun to PeterZ.
Peter, just to clarify, this change is targeted for inclusion in your tree.
Thanks!

> ---
>  kernel/cgroup/cgroup.c | 10 ++++++++++
>  kernel/sched/psi.c     |  4 +---
>  2 files changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index 935e8121b21e..b600a6baaeca 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -3867,6 +3867,12 @@ static __poll_t cgroup_pressure_poll(struct kernfs_open_file *of,
>         return psi_trigger_poll(&ctx->psi.trigger, of->file, pt);
>  }
>
> +static int cgroup_pressure_open(struct kernfs_open_file *of)
> +{
> +       return (of->file->f_mode & FMODE_WRITE && !capable(CAP_SYS_RESOURCE)) ?
> +               -EPERM : 0;
> +}
> +
>  static void cgroup_pressure_release(struct kernfs_open_file *of)
>  {
>         struct cgroup_file_ctx *ctx = of->priv;
> @@ -5266,6 +5272,7 @@ static struct cftype cgroup_psi_files[] = {
>         {
>                 .name = "io.pressure",
>                 .file_offset = offsetof(struct cgroup, psi_files[PSI_IO]),
> +               .open = cgroup_pressure_open,
>                 .seq_show = cgroup_io_pressure_show,
>                 .write = cgroup_io_pressure_write,
>                 .poll = cgroup_pressure_poll,
> @@ -5274,6 +5281,7 @@ static struct cftype cgroup_psi_files[] = {
>         {
>                 .name = "memory.pressure",
>                 .file_offset = offsetof(struct cgroup, psi_files[PSI_MEM]),
> +               .open = cgroup_pressure_open,
>                 .seq_show = cgroup_memory_pressure_show,
>                 .write = cgroup_memory_pressure_write,
>                 .poll = cgroup_pressure_poll,
> @@ -5282,6 +5290,7 @@ static struct cftype cgroup_psi_files[] = {
>         {
>                 .name = "cpu.pressure",
>                 .file_offset = offsetof(struct cgroup, psi_files[PSI_CPU]),
> +               .open = cgroup_pressure_open,
>                 .seq_show = cgroup_cpu_pressure_show,
>                 .write = cgroup_cpu_pressure_write,
>                 .poll = cgroup_pressure_poll,
> @@ -5291,6 +5300,7 @@ static struct cftype cgroup_psi_files[] = {
>         {
>                 .name = "irq.pressure",
>                 .file_offset = offsetof(struct cgroup, psi_files[PSI_IRQ]),
> +               .open = cgroup_pressure_open,
>                 .seq_show = cgroup_irq_pressure_show,
>                 .write = cgroup_irq_pressure_write,
>                 .poll = cgroup_pressure_poll,
> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index 02e011cabe91..0945f956bf80 100644
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -160,7 +160,6 @@ __setup("psi=", setup_psi);
>  #define EXP_300s       2034            /* 1/exp(2s/300s) */
>
>  /* PSI trigger definitions */
> -#define WINDOW_MIN_US 500000   /* Min window size is 500ms */
>  #define WINDOW_MAX_US 10000000 /* Max window size is 10s */
>  #define UPDATES_PER_WINDOW 10  /* 10 updates per window */
>
> @@ -1278,8 +1277,7 @@ struct psi_trigger *psi_trigger_create(struct psi_group *group,
>         if (state >= PSI_NONIDLE)
>                 return ERR_PTR(-EINVAL);
>
> -       if (window_us < WINDOW_MIN_US ||
> -               window_us > WINDOW_MAX_US)
> +       if (window_us == 0 || window_us > WINDOW_MAX_US)
>                 return ERR_PTR(-EINVAL);
>
>         /* Check threshold */
> --
> 2.40.0.rc0.216.gc4246ad0f0-goog
>