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

kan.liang
2020-09-21T15:29:09Z

From: Kan Liang <kan.liang@linux.intel.com>

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@infradead.org>
Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
---
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);
+
+ 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