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From: Mikulas Patocka <mpatocka@redhat.com>
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To: Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: [RFC v2] nvfs: a filesystem for persistent memory
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On Sun, 10 Jan 2021, Al Viro wrote:

> On Sun, Jan 10, 2021 at 04:14:55PM -0500, Mikulas Patocka wrote:
> 
> > That's a good point. I split nvfs_rw_iter to separate functions 
> > nvfs_read_iter and nvfs_write_iter - and inlined nvfs_rw_iter_locked into 
> > both of them. It improved performance by 1.3%.
> > 
> > > Not that it had been more useful on the write side, really,
> > > but that's another story (nvfs_write_pages() handling of
> > > copyin is... interesting).  Let's figure out what's going
> > > on with the read overhead first...
> > > 
> > > lib/iov_iter.c primitives certainly could use massage for
> > > better code generation, but let's find out how much of the
> > > PITA is due to those and how much comes from you fighing
> > > the damn thing instead of using it sanely...
> > 
> > The results are:
> > 
> > read:                                           6.744s
> > read_iter:                                      7.417s
> > read_iter - separate read and write path:       7.321s
> > Al's read_iter:                                 7.182s
> > Al's read_iter with _copy_to_iter:              7.181s
> 
> So
> 	* overhead of hardening stuff is noise here
> 	* switching to more straightforward ->read_iter() cuts
> the overhead by about 1/3.
> 
> 	Interesting...  I wonder how much of that is spent in
> iterate_and_advance() glue inside copy_to_iter() here.  There's
> certainly quite a bit of optimizations possible in those
> primitives and your usecase makes a decent test for that...
> 
> 	Could you profile that and see where is it spending
> the time, on instruction level?

This is the read method profile:

time	9.056s
    52.69%  pread    [kernel.vmlinux]  [k] copy_user_generic_string
     6.24%  pread    [kernel.vmlinux]  [k] current_time
     6.22%  pread    [kernel.vmlinux]  [k] entry_SYSCALL_64
     4.88%  pread    libc-2.31.so      [.] __libc_pread
     3.75%  pread    [kernel.vmlinux]  [k] syscall_return_via_sysret
     3.63%  pread    [nvfs]            [k] nvfs_read
     2.83%  pread    [nvfs]            [k] nvfs_bmap
     2.81%  pread    [kernel.vmlinux]  [k] vfs_read
     2.63%  pread    [kernel.vmlinux]  [k] __x64_sys_pread64
     2.27%  pread    [kernel.vmlinux]  [k] __fsnotify_parent
     2.19%  pread    [kernel.vmlinux]  [k] entry_SYSCALL_64_after_hwframe
     1.55%  pread    [kernel.vmlinux]  [k] atime_needs_update
     1.17%  pread    [kernel.vmlinux]  [k] syscall_enter_from_user_mode
     1.15%  pread    [kernel.vmlinux]  [k] touch_atime
     0.84%  pread    [kernel.vmlinux]  [k] down_read
     0.82%  pread    [kernel.vmlinux]  [k] syscall_exit_to_user_mode
     0.71%  pread    [kernel.vmlinux]  [k] do_syscall_64
     0.68%  pread    [kernel.vmlinux]  [k] ktime_get_coarse_real_ts64
     0.66%  pread    [kernel.vmlinux]  [k] __fget_light
     0.53%  pread    [kernel.vmlinux]  [k] exit_to_user_mode_prepare
     0.45%  pread    [kernel.vmlinux]  [k] up_read
     0.44%  pread    pread             [.] main
     0.44%  pread    [kernel.vmlinux]  [k] syscall_exit_to_user_mode_prepare
     0.26%  pread    [kernel.vmlinux]  [k] entry_SYSCALL_64_safe_stack
     0.12%  pread    pread             [.] pread@plt
     0.07%  pread    [kernel.vmlinux]  [k] __fdget
     0.00%  perf     [kernel.vmlinux]  [k] x86_pmu_enable_all


This is profile of "read_iter - separate read and write path":

time	10.058s
    53.05%  pread    [kernel.vmlinux]  [k] copy_user_generic_string
     6.82%  pread    [kernel.vmlinux]  [k] current_time
     6.27%  pread    [nvfs]            [k] nvfs_read_iter
     4.70%  pread    [kernel.vmlinux]  [k] entry_SYSCALL_64
     3.20%  pread    libc-2.31.so      [.] __libc_pread
     2.77%  pread    [kernel.vmlinux]  [k] syscall_return_via_sysret
     2.31%  pread    [kernel.vmlinux]  [k] vfs_read
     2.15%  pread    [kernel.vmlinux]  [k] new_sync_read
     2.06%  pread    [kernel.vmlinux]  [k] __fsnotify_parent
     2.02%  pread    [nvfs]            [k] nvfs_bmap
     1.87%  pread    [kernel.vmlinux]  [k] entry_SYSCALL_64_after_hwframe
     1.86%  pread    [kernel.vmlinux]  [k] iov_iter_advance
     1.62%  pread    [kernel.vmlinux]  [k] __x64_sys_pread64
     1.40%  pread    [kernel.vmlinux]  [k] atime_needs_update
     0.99%  pread    [kernel.vmlinux]  [k] syscall_enter_from_user_mode
     0.85%  pread    [kernel.vmlinux]  [k] touch_atime
     0.85%  pread    [kernel.vmlinux]  [k] down_read
     0.84%  pread    [kernel.vmlinux]  [k] syscall_exit_to_user_mode
     0.78%  pread    [kernel.vmlinux]  [k] ktime_get_coarse_real_ts64
     0.65%  pread    [kernel.vmlinux]  [k] __fget_light
     0.57%  pread    [kernel.vmlinux]  [k] exit_to_user_mode_prepare
     0.53%  pread    [kernel.vmlinux]  [k] syscall_exit_to_user_mode_prepare
     0.45%  pread    pread             [.] main
     0.43%  pread    [kernel.vmlinux]  [k] up_read
     0.43%  pread    [kernel.vmlinux]  [k] do_syscall_64
     0.28%  pread    [kernel.vmlinux]  [k] iov_iter_init
     0.16%  pread    [kernel.vmlinux]  [k] entry_SYSCALL_64_safe_stack
     0.09%  pread    pread             [.] pread@plt
     0.03%  pread    [kernel.vmlinux]  [k] __fdget
     0.00%  pread    [kernel.vmlinux]  [k] update_rt_rq_load_avg
     0.00%  perf     [kernel.vmlinux]  [k] x86_pmu_enable_all


This is your read_iter_locked profile (read_iter_locked is inlined to 
nvfs_read_iter):

time	10.056s
    50.71%  pread    [kernel.vmlinux]  [k] copy_user_generic_string
     6.95%  pread    [kernel.vmlinux]  [k] current_time
     5.22%  pread    [kernel.vmlinux]  [k] entry_SYSCALL_64
     4.29%  pread    libc-2.31.so      [.] __libc_pread
     4.17%  pread    [nvfs]            [k] nvfs_read_iter
     3.20%  pread    [kernel.vmlinux]  [k] syscall_return_via_sysret
     2.66%  pread    [kernel.vmlinux]  [k] _copy_to_iter
     2.44%  pread    [kernel.vmlinux]  [k] __x64_sys_pread64
     2.38%  pread    [kernel.vmlinux]  [k] new_sync_read
     2.37%  pread    [kernel.vmlinux]  [k] entry_SYSCALL_64_after_hwframe
     2.26%  pread    [kernel.vmlinux]  [k] vfs_read
     2.02%  pread    [nvfs]            [k] nvfs_bmap
     1.88%  pread    [kernel.vmlinux]  [k] __fsnotify_parent
     1.46%  pread    [kernel.vmlinux]  [k] atime_needs_update
     1.08%  pread    [kernel.vmlinux]  [k] touch_atime
     0.83%  pread    [kernel.vmlinux]  [k] syscall_exit_to_user_mode
     0.82%  pread    [kernel.vmlinux]  [k] syscall_enter_from_user_mode
     0.75%  pread    [kernel.vmlinux]  [k] syscall_exit_to_user_mode_prepare
     0.73%  pread    [kernel.vmlinux]  [k] __fget_light
     0.65%  pread    [kernel.vmlinux]  [k] down_read
     0.58%  pread    pread             [.] main
     0.58%  pread    [kernel.vmlinux]  [k] exit_to_user_mode_prepare
     0.52%  pread    [kernel.vmlinux]  [k] ktime_get_coarse_real_ts64
     0.48%  pread    [kernel.vmlinux]  [k] up_read
     0.42%  pread    [kernel.vmlinux]  [k] do_syscall_64
     0.28%  pread    [kernel.vmlinux]  [k] iov_iter_init
     0.13%  pread    [kernel.vmlinux]  [k] __fdget
     0.12%  pread    [kernel.vmlinux]  [k] entry_SYSCALL_64_safe_stack
     0.03%  pread    pread             [.] pread@plt
     0.00%  perf     [kernel.vmlinux]  [k] x86_pmu_enable_all

Mikulas
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To:     Al Viro <viro@zeniv.linux.org.uk>
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Subject: Re: [RFC v2] nvfs: a filesystem for persistent memory
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On Sun, 10 Jan 2021, Al Viro wrote:

> On Sun, Jan 10, 2021 at 04:14:55PM -0500, Mikulas Patocka wrote:
> 
> > That's a good point. I split nvfs_rw_iter to separate functions 
> > nvfs_read_iter and nvfs_write_iter - and inlined nvfs_rw_iter_locked into 
> > both of them. It improved performance by 1.3%.
> > 
> > > Not that it had been more useful on the write side, really,
> > > but that's another story (nvfs_write_pages() handling of
> > > copyin is... interesting).  Let's figure out what's going
> > > on with the read overhead first...
> > > 
> > > lib/iov_iter.c primitives certainly could use massage for
> > > better code generation, but let's find out how much of the
> > > PITA is due to those and how much comes from you fighing
> > > the damn thing instead of using it sanely...
> > 
> > The results are:
> > 
> > read:                                           6.744s
> > read_iter:                                      7.417s
> > read_iter - separate read and write path:       7.321s
> > Al's read_iter:                                 7.182s
> > Al's read_iter with _copy_to_iter:              7.181s
> 
> So
> 	* overhead of hardening stuff is noise here
> 	* switching to more straightforward ->read_iter() cuts
> the overhead by about 1/3.
> 
> 	Interesting...  I wonder how much of that is spent in
> iterate_and_advance() glue inside copy_to_iter() here.  There's
> certainly quite a bit of optimizations possible in those
> primitives and your usecase makes a decent test for that...
> 
> 	Could you profile that and see where is it spending
> the time, on instruction level?

This is the read method profile:

time	9.056s
    52.69%  pread    [kernel.vmlinux]  [k] copy_user_generic_string
     6.24%  pread    [kernel.vmlinux]  [k] current_time
     6.22%  pread    [kernel.vmlinux]  [k] entry_SYSCALL_64
     4.88%  pread    libc-2.31.so      [.] __libc_pread
     3.75%  pread    [kernel.vmlinux]  [k] syscall_return_via_sysret
     3.63%  pread    [nvfs]            [k] nvfs_read
     2.83%  pread    [nvfs]            [k] nvfs_bmap
     2.81%  pread    [kernel.vmlinux]  [k] vfs_read
     2.63%  pread    [kernel.vmlinux]  [k] __x64_sys_pread64
     2.27%  pread    [kernel.vmlinux]  [k] __fsnotify_parent
     2.19%  pread    [kernel.vmlinux]  [k] entry_SYSCALL_64_after_hwframe
     1.55%  pread    [kernel.vmlinux]  [k] atime_needs_update
     1.17%  pread    [kernel.vmlinux]  [k] syscall_enter_from_user_mode
     1.15%  pread    [kernel.vmlinux]  [k] touch_atime
     0.84%  pread    [kernel.vmlinux]  [k] down_read
     0.82%  pread    [kernel.vmlinux]  [k] syscall_exit_to_user_mode
     0.71%  pread    [kernel.vmlinux]  [k] do_syscall_64
     0.68%  pread    [kernel.vmlinux]  [k] ktime_get_coarse_real_ts64
     0.66%  pread    [kernel.vmlinux]  [k] __fget_light
     0.53%  pread    [kernel.vmlinux]  [k] exit_to_user_mode_prepare
     0.45%  pread    [kernel.vmlinux]  [k] up_read
     0.44%  pread    pread             [.] main
     0.44%  pread    [kernel.vmlinux]  [k] syscall_exit_to_user_mode_prepare
     0.26%  pread    [kernel.vmlinux]  [k] entry_SYSCALL_64_safe_stack
     0.12%  pread    pread             [.] pread@plt
     0.07%  pread    [kernel.vmlinux]  [k] __fdget
     0.00%  perf     [kernel.vmlinux]  [k] x86_pmu_enable_all


This is profile of "read_iter - separate read and write path":

time	10.058s
    53.05%  pread    [kernel.vmlinux]  [k] copy_user_generic_string
     6.82%  pread    [kernel.vmlinux]  [k] current_time
     6.27%  pread    [nvfs]            [k] nvfs_read_iter
     4.70%  pread    [kernel.vmlinux]  [k] entry_SYSCALL_64
     3.20%  pread    libc-2.31.so      [.] __libc_pread
     2.77%  pread    [kernel.vmlinux]  [k] syscall_return_via_sysret
     2.31%  pread    [kernel.vmlinux]  [k] vfs_read
     2.15%  pread    [kernel.vmlinux]  [k] new_sync_read
     2.06%  pread    [kernel.vmlinux]  [k] __fsnotify_parent
     2.02%  pread    [nvfs]            [k] nvfs_bmap
     1.87%  pread    [kernel.vmlinux]  [k] entry_SYSCALL_64_after_hwframe
     1.86%  pread    [kernel.vmlinux]  [k] iov_iter_advance
     1.62%  pread    [kernel.vmlinux]  [k] __x64_sys_pread64
     1.40%  pread    [kernel.vmlinux]  [k] atime_needs_update
     0.99%  pread    [kernel.vmlinux]  [k] syscall_enter_from_user_mode
     0.85%  pread    [kernel.vmlinux]  [k] touch_atime
     0.85%  pread    [kernel.vmlinux]  [k] down_read
     0.84%  pread    [kernel.vmlinux]  [k] syscall_exit_to_user_mode
     0.78%  pread    [kernel.vmlinux]  [k] ktime_get_coarse_real_ts64
     0.65%  pread    [kernel.vmlinux]  [k] __fget_light
     0.57%  pread    [kernel.vmlinux]  [k] exit_to_user_mode_prepare
     0.53%  pread    [kernel.vmlinux]  [k] syscall_exit_to_user_mode_prepare
     0.45%  pread    pread             [.] main
     0.43%  pread    [kernel.vmlinux]  [k] up_read
     0.43%  pread    [kernel.vmlinux]  [k] do_syscall_64
     0.28%  pread    [kernel.vmlinux]  [k] iov_iter_init
     0.16%  pread    [kernel.vmlinux]  [k] entry_SYSCALL_64_safe_stack
     0.09%  pread    pread             [.] pread@plt
     0.03%  pread    [kernel.vmlinux]  [k] __fdget
     0.00%  pread    [kernel.vmlinux]  [k] update_rt_rq_load_avg
     0.00%  perf     [kernel.vmlinux]  [k] x86_pmu_enable_all


This is your read_iter_locked profile (read_iter_locked is inlined to 
nvfs_read_iter):

time	10.056s
    50.71%  pread    [kernel.vmlinux]  [k] copy_user_generic_string
     6.95%  pread    [kernel.vmlinux]  [k] current_time
     5.22%  pread    [kernel.vmlinux]  [k] entry_SYSCALL_64
     4.29%  pread    libc-2.31.so      [.] __libc_pread
     4.17%  pread    [nvfs]            [k] nvfs_read_iter
     3.20%  pread    [kernel.vmlinux]  [k] syscall_return_via_sysret
     2.66%  pread    [kernel.vmlinux]  [k] _copy_to_iter
     2.44%  pread    [kernel.vmlinux]  [k] __x64_sys_pread64
     2.38%  pread    [kernel.vmlinux]  [k] new_sync_read
     2.37%  pread    [kernel.vmlinux]  [k] entry_SYSCALL_64_after_hwframe
     2.26%  pread    [kernel.vmlinux]  [k] vfs_read
     2.02%  pread    [nvfs]            [k] nvfs_bmap
     1.88%  pread    [kernel.vmlinux]  [k] __fsnotify_parent
     1.46%  pread    [kernel.vmlinux]  [k] atime_needs_update
     1.08%  pread    [kernel.vmlinux]  [k] touch_atime
     0.83%  pread    [kernel.vmlinux]  [k] syscall_exit_to_user_mode
     0.82%  pread    [kernel.vmlinux]  [k] syscall_enter_from_user_mode
     0.75%  pread    [kernel.vmlinux]  [k] syscall_exit_to_user_mode_prepare
     0.73%  pread    [kernel.vmlinux]  [k] __fget_light
     0.65%  pread    [kernel.vmlinux]  [k] down_read
     0.58%  pread    pread             [.] main
     0.58%  pread    [kernel.vmlinux]  [k] exit_to_user_mode_prepare
     0.52%  pread    [kernel.vmlinux]  [k] ktime_get_coarse_real_ts64
     0.48%  pread    [kernel.vmlinux]  [k] up_read
     0.42%  pread    [kernel.vmlinux]  [k] do_syscall_64
     0.28%  pread    [kernel.vmlinux]  [k] iov_iter_init
     0.13%  pread    [kernel.vmlinux]  [k] __fdget
     0.12%  pread    [kernel.vmlinux]  [k] entry_SYSCALL_64_safe_stack
     0.03%  pread    pread             [.] pread@plt
     0.00%  perf     [kernel.vmlinux]  [k] x86_pmu_enable_all

Mikulas