On Fri 10-09-21 15:44:00, Feng Tang wrote:
> On Wed, Sep 08, 2021 at 09:06:24AM +0200, Michal Hocko wrote:
> > On Wed 08-09-21 09:50:14, Feng Tang wrote:
> > > On Tue, Sep 07, 2021 at 10:44:32AM +0200, Michal Hocko wrote:
> > [...]
> > > > While this is a good fix from the functionality POV I believe you can go
> > > > a step further. Please add a detection to the cpuset code and complain
> > > > to the kernel log if somebody tries to configure movable only cpuset.
> > > > Once you have that in place you can easily create a static branch for
> > > > cpuset_insane_setup() and have zero overhead for all reasonable
> > > > configuration. There shouldn't be any reason to pay a single cpu cycle
> > > > to check for something that almost nobody does.
> > > >
> > > > What do you think?
> > >
> > > I thought about the implementation, IIUC, the static_branch_enable() is
> > > easy, it could be done when cpuset.mems is set with movable only nodes,
> > > but disable() is much complexer,
> >
> > Do we care about disable at all? The point is to not have 99,999999%
> > users pay overhead of the check which is irrelevant to them. Once
> > somebody wants to use this "creative" setup then paying an extra check
> > sounds perfectly sensible to me. If somebody cares enough then the
> > disable logic could be implemented. But for now I believe we should be
> > OK with only enable case.
>
> Here is tested draft patch to add the check in cpuset code (the looping
> zone code could be improved by adding a for_each_populated_zone_nodemask
> macro.
>
> Thanks,
> Feng
>
> ---
> include/linux/cpuset.h | 7 +++++++
> include/linux/mmzone.h | 14 ++++++++++++++
> kernel/cgroup/cpuset.c | 10 ++++++++++
> mm/page_alloc.c | 4 +++-
> 4 files changed, 34 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
> index d2b9c41..a434985 100644
> --- a/include/linux/cpuset.h
> +++ b/include/linux/cpuset.h
> @@ -34,6 +34,8 @@
> */
> extern struct static_key_false cpusets_pre_enable_key;
> extern struct static_key_false cpusets_enabled_key;
> +extern struct static_key_false cpusets_abnormal_setup_key;
> +
> static inline bool cpusets_enabled(void)
> {
> return static_branch_unlikely(&cpusets_enabled_key);
> @@ -51,6 +53,11 @@ static inline void cpuset_dec(void)
> static_branch_dec_cpuslocked(&cpusets_pre_enable_key);
> }
>
> +static inline bool cpusets_abnormal_check_needed(void)
I would go with cpusets_insane_config with a comment explaining what
that means. I would also do a pr_info() when the static branch is
enabled.
[...]
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 4e455fa..5728675 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4919,7 +4919,9 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> * any suitable zone to satisfy the request - e.g. non-movable
> * GFP_HIGHUSER allocations from MOVABLE nodes only.
> */
> - if (cpusets_enabled() && (gfp_mask & __GFP_HARDWALL)) {
> + if (cpusets_enabled() &&
> + cpusets_abnormal_check_needed() &&
You do not need cpusets_enabled check here. Remember the primary point
is to not introduce any branch unless a dubious configuration is in
place.
> + (gfp_mask & __GFP_HARDWALL)) {
> struct zoneref *z = first_zones_zonelist(ac->zonelist,
> ac->highest_zoneidx,
> &cpuset_current_mems_allowed);
> --
> 2.7.4
>
--
Michal Hocko
SUSE Labs
