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ecdfb8896f
We can still see that a majority of the time is spent hashing task pointers:
...
16.98% [kernel] [k] rhashtable_jhash2
...
Doing the bookkeeping in toggle_bp_slots() is currently O(#cpus),
calling task_bp_pinned() for each CPU, even if task_bp_pinned() is
CPU-independent. The reason for this is to update the per-CPU
'tsk_pinned' histogram.
To optimize the CPU-independent case to O(1), keep a separate
CPU-independent 'tsk_pinned_all' histogram.
The major source of complexity are transitions between "all
CPU-independent task breakpoints" and "mixed CPU-independent and
CPU-dependent task breakpoints". The code comments list all cases that
require handling.
After this optimization:
| $> perf bench -r 100 breakpoint thread -b 4 -p 128 -t 512
| # Running 'breakpoint/thread' benchmark:
| # Created/joined 100 threads with 4 breakpoints and 128 parallelism
| Total time: 1.758 [sec]
|
| 34.336621 usecs/op
| 4395.087500 usecs/op/cpu
38.08% [kernel] [k] queued_spin_lock_slowpath
10.81% [kernel] [k] smp_cfm_core_cond
3.01% [kernel] [k] update_sg_lb_stats
2.58% [kernel] [k] osq_lock
2.57% [kernel] [k] llist_reverse_order
1.45% [kernel] [k] find_next_bit
1.21% [kernel] [k] flush_tlb_func_common
1.01% [kernel] [k] arch_install_hw_breakpoint
Showing that the time spent hashing keys has become insignificant.
With the given benchmark parameters, that's an improvement of 12%
compared with the old O(#cpus) version.
And finally, using the less aggressive parameters from the preceding
changes, we now observe:
| $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
| # Running 'breakpoint/thread' benchmark:
| # Created/joined 30 threads with 4 breakpoints and 64 parallelism
| Total time: 0.067 [sec]
|
| 35.292187 usecs/op
| 2258.700000 usecs/op/cpu
Which is an improvement of 12% compared to without the histogram
optimizations (baseline is 40 usecs/op). This is now on par with the
theoretical ideal (constraints disabled), and only 12% slower than no
breakpoints at all.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Acked-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20220829124719.675715-15-elver@google.com
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.