If a process (qemu) with a lot of CPUs (128) try to munmap() a large
chunk of memory (496GB) mapped with THP, it takes an average of 275
seconds, which can cause a lot of problems to the load (in qemu case,
the guest will lock for this time).
Trying to find the source of this bug, I found out most of this time is
spent on serialize_against_pte_lookup(). This function will take a lot
of time in smp_call_function_many() if there is more than a couple CPUs
running the user process. Since it has to happen to all THP mapped, it
will take a very long time for large amounts of memory.
By the docs, serialize_against_pte_lookup() is needed in order to avoid
pmd_t to pte_t casting inside find_current_mm_pte(), or any lockless
pagetable walk, to happen concurrently with THP splitting/collapsing.
It does so by calling a do_nothing() on each CPU in mm->cpu_bitmap[],
after interrupts are re-enabled.
Since, interrupts are (usually) disabled during lockless pagetable
walk, and serialize_against_pte_lookup will only return after
interrupts are enabled, it is protected.
So, by what I could understand, if there is no lockless pagetable walk
running, there is no need to call serialize_against_pte_lookup().
So, to avoid the cost of running serialize_against_pte_lookup(), I
propose a counter that keeps track of how many find_current_mm_pte()
are currently running, and if there is none, just skip
smp_call_function_many().
The related functions are:
begin_lockless_pgtbl_walk()
Insert before starting any lockless pgtable walk
end_lockless_pgtbl_walk()
Insert after the end of any lockless pgtable walk
(Mostly after the ptep is last used)
running_lockless_pgtbl_walk()
Returns the number of lockless pgtable walks running
On my workload (qemu), I could see munmap's time reduction from 275
seconds to 418ms.
Also, I documented some lockless pagetable walks in which it's not
necessary to keep track, given they work on init_mm or guest pgd.
The patchset works by focusing all steps needed to begin/end lockless
pagetable walks on the above functions, and then adding the config option
to enable the tracking of these functions using the counting method.
Changes since v4:
Rebased on top of v5.4-rc1
Declared real generic functions instead of dummies
start_lockless_pgtbl_walk renamed to begin_lockless_pgtbl_walk
Interrupt {dis,en}able is now inside of {begin,end}_lockless_pgtbl_walk
Power implementation has option to not {dis,en}able interrupt
More documentation inside the funtions.
Some irq maks variables renamed
Removed some proxy mm_structs
Few typos fixed
Changes since v3:
Explain (comments) why some lockless pgtbl walks don't need
local_irq_disable (real mode + MSR_EE=0)
Explain (comments) places where counting method is not needed (guest pgd,
which is not touched by THP)
Fixes some misplaced local_irq_restore()
Link: http://patchwork.ozlabs.org/project/linuxppc-dev/list/?series=132417
Changes since v2:
Rebased to v5.3
Adds support on __get_user_pages_fast
Adds usage decription to *_lockless_pgtbl_walk()
Better style to dummy functions
Link: http://patchwork.ozlabs.org/project/linuxppc-dev/list/?series=131839
Changes since v1:
Isolated atomic operations in functions *_lockless_pgtbl_walk()
Fixed behavior of decrementing before last ptep was used
Link: http://patchwork.ozlabs.org/patch/1163093/
Leonardo Bras (11):
asm-generic/pgtable: Adds generic functions to monitor lockless
pgtable walks
powerpc/mm: Adds counting method to monitor lockless pgtable walks
mm/gup: Applies counting method to monitor gup_pgd_range
powerpc/mce_power: Applies counting method to monitor lockless pgtbl
walks
powerpc/perf: Applies counting method to monitor lockless pgtbl walks
powerpc/mm/book3s64/hash: Applies counting method to monitor lockless
pgtbl walks
powerpc/kvm/e500: Applies counting method to monitor lockless pgtbl
walks
powerpc/kvm/book3s_hv: Applies counting method to monitor lockless
pgtbl walks
powerpc/kvm/book3s_64: Applies counting method to monitor lockless
pgtbl walks
mm/Kconfig: Adds config option to track lockless pagetable walks
powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing
arch/powerpc/include/asm/book3s/64/pgtable.h | 9 ++
arch/powerpc/kernel/mce_power.c | 6 +-
arch/powerpc/kvm/book3s_64_mmu_hv.c | 6 +-
arch/powerpc/kvm/book3s_64_mmu_radix.c | 34 +++++-
arch/powerpc/kvm/book3s_64_vio_hv.c | 7 +-
arch/powerpc/kvm/book3s_hv_nested.c | 22 +++-
arch/powerpc/kvm/book3s_hv_rm_mmu.c | 32 ++---
arch/powerpc/kvm/e500_mmu_host.c | 9 +-
arch/powerpc/mm/book3s64/hash_tlb.c | 6 +-
arch/powerpc/mm/book3s64/hash_utils.c | 27 +++--
arch/powerpc/mm/book3s64/pgtable.c | 120 ++++++++++++++++++-
arch/powerpc/perf/callchain.c | 6 +-
include/asm-generic/pgtable.h | 58 +++++++++
include/linux/mm_types.h | 11 ++
kernel/fork.c | 3 +
mm/Kconfig | 11 ++
mm/gup.c | 10 +-
17 files changed, 325 insertions(+), 52 deletions(-)
--
2.20.1
