Re: [PATCH 2/2] arm64/mm: Enable color zero pages

From: Robin Murphy
Date: Thu Sep 17 2020 - 06:31:34 EST

On 2020-09-17 04:35, Gavin Shan wrote:
Hi Will,

On 9/16/20 6:28 PM, Will Deacon wrote:
On Wed, Sep 16, 2020 at 01:25:23PM +1000, Gavin Shan wrote:
This enables color zero pages by allocating contigous page frames
for it. The number of pages for this is determined by L1 dCache
(or iCache) size, which is probbed from the hardware.

    * Add cache_total_size() to return L1 dCache (or iCache) size

    * Implement setup_zero_pages(), which is called after the page
      allocator begins to work, to allocate the contigous pages
      needed by color zero page.

    * Reworked ZERO_PAGE() and define __HAVE_COLOR_ZERO_PAGE.

Signed-off-by: Gavin Shan <gshan@xxxxxxxxxx>
  arch/arm64/include/asm/cache.h   | 22 ++++++++++++++++++++
  arch/arm64/include/asm/pgtable.h |  9 ++++++--
  arch/arm64/kernel/cacheinfo.c    | 34 +++++++++++++++++++++++++++++++
  arch/arm64/mm/init.c             | 35 ++++++++++++++++++++++++++++++++
  arch/arm64/mm/mmu.c              |  7 -------
  5 files changed, 98 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/include/asm/cache.h b/arch/arm64/include/asm/cache.h
index a4d1b5f771f6..420e9dde2c51 100644
--- a/arch/arm64/include/asm/cache.h
+++ b/arch/arm64/include/asm/cache.h
@@ -39,6 +39,27 @@
  #define CLIDR_LOC(clidr)    (((clidr) >> CLIDR_LOC_SHIFT) & 0x7)
  #define CLIDR_LOUIS(clidr)    (((clidr) >> CLIDR_LOUIS_SHIFT) & 0x7)
+#define CSSELR_TND_SHIFT    4
+#define CSSELR_TND_MASK        (UL(1) << CSSELR_TND_SHIFT)
+#define CSSELR_LEVEL_SHIFT    1
+#define CSSELR_IND_SHIFT    0
+#define CSSERL_IND_MASK        (UL(1) << CSSELR_IND_SHIFT)
+#define CCSIDR_64_LS_SHIFT    0
+#define CCSIDR_64_LS_MASK    (UL(7) << CCSIDR_64_LS_SHIFT)
+#define CCSIDR_64_ASSOC_SHIFT    3
+#define CCSIDR_64_SET_SHIFT    32
+#define CCSIDR_32_LS_SHIFT    0
+#define CCSIDR_32_LS_MASK    (UL(7) << CCSIDR_32_LS_SHIFT)
+#define CCSIDR_32_ASSOC_SHIFT    3
+#define CCSIDR_32_SET_SHIFT    13
+#define CCSIDR_32_SET_MASK    (UL(0x7FFF) << CCSIDR_32_SET_SHIFT)

I don't think we should be inferring cache structure from these register
values. The Arm ARM helpfully says:

   | You cannot make any inference about the actual sizes of caches based
   | on these parameters.

so we need to take the topology information from elsewhere.

Yeah, I also noticed the statement in the spec. However, the L1 cache size
figured out from above registers are matching with "lscpu" on the machine
where I did my tests. Note "lscpu" depends on sysfs entries whose information
is retrieved from ACPI (PPTT) table. The number of cache levels are partially
retrieved from system register (clidr_el1).

It's doable to retrieve the L1 cache size from ACPI (PPTT) table. I'll
change accordingly in v2 if this enablement is really needed. More clarify
is provided below.

But before we get into that, can you justify why we need to do this at all,
please? Do you have data to show the benefit of adding this complexity?

Initially, I found it's the missed feature which has been enabled on
mips/s390. Currently, all read-only anonymous VMAs are backed up by
same zero page. It means all reads to these VMAs are cached by same
set of cache, but still multiple ways if supported. So it would be
nice to have multiple zero pages to back up these read-only anonymous
VMAs, so that the reads on them can be cached by multiple sets (multiple
ways still if supported). It's overall beneficial to the performance.

Is this a concern for true PIPT caches, or is it really just working around a pathological case for alias-detecting VIPT caches?

Unfortunately, I didn't find a machine where the size of cache set is
larger than page size. So I had one experiment as indication how L1
data cache miss affects the overall performance:

    L1 data cache size:           32KB
    L1 data cache line size:      64
    Number of L1 data cache set:  64
    Number of L1 data cache ways: 8
                  size = (cache_line_size) * (num_of_sets) * (num_of_ways)

    Kernel configuration:
           VA_BITS:               48
           PAGE_SIZE:             4KB
           PMD HugeTLB Page Size: 2MB

           I have a program to do the following things and check the
           consumed time and L1-data-cache-misses by perf.

           (1) Allocate (mmap) a PMD HugeTLB Page, which is 2MB.
           (2) Read on the mmap'd region in step of page size (4KB)
               for 8 or 9 times. Note 8 is the number of data cache
           (3) Repeat (2) for 1000000 times.
           (a) when we have 8 for the steps in (2):
               37,103      L1-dcache-load-misses
               0.217522515 seconds time elapsed
               0.217564000 seconds user
               0.000000000 seconds sys
           (b) when we have 9 for the steps in (2):
               4,687,932   L1-dcache-load-misses            (126 times)
               0.248132105 seconds time elapsed             (+14.2%)
               0.248267000 seconds user
               0.000000000 seconds sys

I have a vague feeling this may have come up before, but are there real-world applications that have a performance bottleneck on reading from uninitialised memory? As far as synthetic benchmarks go, I'm sure we could equally come up with one that shows a regression due to real data being pushed out of the cache by all those extra zeros ;)


Please let me know if it's worthy for a v2, to retrieve the cache size
from ACPI (PPTT) table. The cost is to allocate multiple zero pages and
the worst case is fail back to one zero page, as before :)


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