Re: [PATCH V8 1/4] perf/core: Add PERF_SAMPLE_DATA_PAGE_SIZE

[Stephane Eranian]

On Mon, Sep 21, 2020 at 8:29 AM <kan.liang@xxxxxxxxxxxxxxx> wrote:
>
> From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
>
> Current perf can report both virtual addresses and physical addresses,
> but not the MMU page size. Without the MMU page size information of the
> utilized page, users cannot decide whether to promote/demote large pages
> to optimize memory usage.
>
> Add a new sample type for the data MMU page size.
>
> Current perf already has a facility to collect data virtual addresses.
> A page walker is required to walk the pages tables and calculate the
> MMU page size from a given virtual address.
>
> On some platforms, e.g., X86, the page walker is invoked in an NMI
> handler. So the page walker must be NMI-safe and low overhead. Besides,
> the page walker should work for both user and kernel virtual address.
> The existing generic page walker, e.g., walk_page_range_novma(), is a
> little bit complex and doesn't guarantee the NMI-safe. The follow_page()
> is only for user-virtual address.
>
> Add a new function perf_get_page_size() to walk the page tables and
> calculate the MMU page size. In the function:
> - Interrupts have to be disabled to prevent any teardown of the page
>   tables.
> - The active_mm is used for the page walker. Compared with mm, the
>   active_mm is a better choice. It's always non-NULL. For the user
>   thread, it always points to the real address space. For the kernel
>   thread, it "take over" the mm of the threads that switched to it,
>   so it's not using all of the page tables from the init_mm all the
>   time.
> - The MMU page size is calculated from the page table level.
>
> The method should work for all architectures, but it has only been
> verified on X86. Should there be some architectures, which support perf,
> where the method doesn't work, it can be fixed later separately.
> Reporting the wrong page size would not be fatal for the architecture.
>
> Some under discussion features may impact the method in the future.
> Quote from Dave Hansen,
>   "There are lots of weird things folks are trying to do with the page
>    tables, like Address Space Isolation.  For instance, if you get a
>    perf NMI when running userspace, current->mm->pgd is *different* than
>    the PGD that was in use when userspace was running. It's close enough
>    today, but it might not stay that way."
> If the case happens later, lots of consecutive page walk errors will
> happen. The worst case is that lots of page-size '0' are returned, which
> would not be fatal.
> In the perf tool, a check is implemented to detect this case. Once it
> happens, a kernel patch could be implemented accordingly then.
>
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> ---
>  include/linux/perf_event.h      |  1 +
>  include/uapi/linux/perf_event.h |  4 +-
>  kernel/events/core.c            | 93 +++++++++++++++++++++++++++++++++
>  3 files changed, 97 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 0c19d279b97f..7e3785dd27d9 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -1034,6 +1034,7 @@ struct perf_sample_data {
>
>         u64                             phys_addr;
>         u64                             cgroup;
> +       u64                             data_page_size;
>  } ____cacheline_aligned;
>
>  /* default value for data source */
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index 077e7ee69e3d..cc6ea346e9f9 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -143,8 +143,9 @@ enum perf_event_sample_format {
>         PERF_SAMPLE_PHYS_ADDR                   = 1U << 19,
>         PERF_SAMPLE_AUX                         = 1U << 20,
>         PERF_SAMPLE_CGROUP                      = 1U << 21,
> +       PERF_SAMPLE_DATA_PAGE_SIZE              = 1U << 22,
>
> -       PERF_SAMPLE_MAX = 1U << 22,             /* non-ABI */
> +       PERF_SAMPLE_MAX = 1U << 23,             /* non-ABI */
>
>         __PERF_SAMPLE_CALLCHAIN_EARLY           = 1ULL << 63, /* non-ABI; internal use */
>  };
> @@ -896,6 +897,7 @@ enum perf_event_type {
>          *      { u64                   phys_addr;} && PERF_SAMPLE_PHYS_ADDR
>          *      { u64                   size;
>          *        char                  data[size]; } && PERF_SAMPLE_AUX
> +        *      { u64                   data_page_size;} && PERF_SAMPLE_DATA_PAGE_SIZE
>          * };
>          */
>         PERF_RECORD_SAMPLE                      = 9,
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 45edb85344a1..dd329a8f99f7 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -51,6 +51,7 @@
>  #include <linux/proc_ns.h>
>  #include <linux/mount.h>
>  #include <linux/min_heap.h>
> +#include <linux/highmem.h>
>
>  #include "internal.h"
>
> @@ -1894,6 +1895,9 @@ static void __perf_event_header_size(struct perf_event *event, u64 sample_type)
>         if (sample_type & PERF_SAMPLE_CGROUP)
>                 size += sizeof(data->cgroup);
>
> +       if (sample_type & PERF_SAMPLE_DATA_PAGE_SIZE)
> +               size += sizeof(data->data_page_size);
> +
>         event->header_size = size;
>  }
>
> @@ -6937,6 +6941,9 @@ void perf_output_sample(struct perf_output_handle *handle,
>         if (sample_type & PERF_SAMPLE_CGROUP)
>                 perf_output_put(handle, data->cgroup);
>
> +       if (sample_type & PERF_SAMPLE_DATA_PAGE_SIZE)
> +               perf_output_put(handle, data->data_page_size);
> +
>         if (sample_type & PERF_SAMPLE_AUX) {
>                 perf_output_put(handle, data->aux_size);
>
> @@ -6994,6 +7001,84 @@ static u64 perf_virt_to_phys(u64 virt)
>         return phys_addr;
>  }
>
> +#ifdef CONFIG_MMU
> +
> +/*
> + * Return the MMU page size of a given virtual address
> + */
> +static u64 __perf_get_page_size(struct mm_struct *mm, unsigned long addr)
> +{
> +       pgd_t *pgd;
> +       p4d_t *p4d;
> +       pud_t *pud;
> +       pmd_t *pmd;
> +       pte_t *pte;
> +
> +       pgd = pgd_offset(mm, addr);
> +       if (pgd_none(*pgd))
> +               return 0;
> +
> +       p4d = p4d_offset(pgd, addr);
> +       if (!p4d_present(*p4d))
> +               return 0;
> +
> +       if (p4d_leaf(*p4d))
> +               return 1ULL << P4D_SHIFT;
> +
> +       pud = pud_offset(p4d, addr);
> +       if (!pud_present(*pud))
> +               return 0;
> +
> +       if (pud_leaf(*pud))
> +               return 1ULL << PUD_SHIFT;
> +
> +       pmd = pmd_offset(pud, addr);
> +       if (!pmd_present(*pmd))
> +               return 0;
> +
> +       if (pmd_leaf(*pmd))
> +               return 1ULL << PMD_SHIFT;
> +
> +       pte = pte_offset_map(pmd, addr);
> +       if (!pte_present(*pte)) {
> +               pte_unmap(pte);
> +               return 0;
> +       }
> +
> +       pte_unmap(pte);
> +       return PAGE_SIZE;
> +}
> +
> +#else
> +
> +static u64 __perf_get_page_size(struct mm_struct *mm, unsigned long addr)
> +{
> +       return 0;
> +}
> +
> +#endif
> +
> +static u64 perf_get_page_size(unsigned long addr)
> +{
> +       unsigned long flags;
> +       u64 size;
> +
> +       if (!addr)
> +               return 0;
> +
> +       /*
> +        * Software page-table walkers must disable IRQs,
> +        * which prevents any tear down of the page tables.
> +        */
> +       local_irq_save(flags);
> +
> +       size = __perf_get_page_size(current->active_mm, addr);
> +

When I tested on my kernel, it panicked because I suspect
current->active_mm could be NULL. Adding a check for NULL avoided the
problem. But I suspect this is not the correct solution.

>
> +       local_irq_restore(flags);
> +
> +       return size;
> +}
> +
>  static struct perf_callchain_entry __empty_callchain = { .nr = 0, };
>
>  struct perf_callchain_entry *
> @@ -7149,6 +7234,14 @@ void perf_prepare_sample(struct perf_event_header *header,
>         }
>  #endif
>
> +       /*
> +        * PERF_DATA_PAGE_SIZE requires PERF_SAMPLE_ADDR. If the user doesn't
> +        * require PERF_SAMPLE_ADDR, kernel implicitly retrieve the data->addr,
> +        * but the value will not dump to the userspace.
> +        */
> +       if (sample_type & PERF_SAMPLE_DATA_PAGE_SIZE)
> +               data->data_page_size = perf_get_page_size(data->addr);
> +
>         if (sample_type & PERF_SAMPLE_AUX) {
>                 u64 size;
>
> --
> 2.17.1
>