Re: [RFC PATCH v1 16/57] perf: Remove PAGE_SIZE compile-time constant assumption

Ryan Roberts
2024-10-16T14:44:23Z

+ Arnaldo Carvalho de Melo, Ingo Molnar, Namhyung Kim, Peter Zijlstra

This was a rather tricky series to get the recipients correct for and my script
did not realize that "supporter" was a pseudonym for "maintainer" so you were
missed off the original post. Appologies!

More context in cover letter:
https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@arm.com/

On 14/10/2024 11:58, Ryan Roberts wrote:
> To prepare for supporting boot-time page size selection, refactor code
> to remove assumptions about PAGE_SIZE being compile-time constant. Code
> intended to be equivalent when compile-time page size is active.
>
> Refactor a BUILD_BUG_ON() so that we test against the limit; _format is
> invariant to page size so testing it is no bigger than the minimum
> supported size is sufficient.
>
> Wrap global variables that are initialized with PAGE_SIZE derived values
> using DEFINE_GLOBAL_PAGE_SIZE_VAR() so their initialization can be
> deferred for boot-time page size builds.
>
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
>
> ***NOTE***
> Any confused maintainers may want to read the cover note here for context:
> https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@arm.com/
>
> include/linux/perf_event.h | 2 +-
> kernel/events/core.c | 2 +-
> 2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 1a8942277ddad..b7972155f93eb 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -1872,7 +1872,7 @@ _name##_show(struct device *dev, \
> struct device_attribute *attr, \
> char *page) \
> { \
> - BUILD_BUG_ON(sizeof(_format) >= PAGE_SIZE); \
> + BUILD_BUG_ON(sizeof(_format) >= PAGE_SIZE_MIN); \
> return sprintf(page, _format "\n"); \
> } \
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 8a6c6bbcd658a..81149663ab7d8 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -419,7 +419,7 @@ static struct kmem_cache *perf_event_cache;
> int sysctl_perf_event_paranoid __read_mostly = 2;
>
> /* Minimum for 512 kiB + 1 user control page */
> -int sysctl_perf_event_mlock __read_mostly = 512 + (PAGE_SIZE / 1024); /* 'free' kiB per user */
> +__DEFINE_GLOBAL_PAGE_SIZE_VAR(int, sysctl_perf_event_mlock, __read_mostly, 512 + (PAGE_SIZE / 1024)); /* 'free' kiB per user */
>
> /*
> * max perf event sample rate