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Song Liu noticed switch_mm_irqs_off() taking a lot of CPU time in recent kernels,using 1.8% of a 48 CPU system during a netperf to localhost run. Digging into the profile, we noticed that cpumask_clear_cpu and cpumask_set_cpu together take about half of the CPU time taken by switch_mm_irqs_off(). However, the CPUs running netperf end up switching back and forth between netperf and the idle task, which does not require changes to the mm_cpumask. Furthermore, the init_mm cpumask ends up being the most heavily contended one in the system. Simply skipping changes to mm_cpumask(&init_mm) reduces overhead. Reported-and-tested-by: Song Liu <songliubraving@fb.com> Signed-off-by: Rik van Riel <riel@surriel.com> Acked-by: Dave Hansen <dave.hansen@intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: efault@gmx.de Cc: kernel-team@fb.com Cc: luto@kernel.org Link: http://lkml.kernel.org/r/20180716190337.26133-8-riel@surriel.com Signed-off-by: Ingo Molnar <mingo@kernel.org> |
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| fs | ||
| include | ||
| init | ||
| ipc | ||
| kernel | ||
| lib | ||
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| net | ||
| samples | ||
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| .get_maintainer.ignore | ||
| .gitattributes | ||
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| COPYING | ||
| CREDITS | ||
| Kbuild | ||
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| README | ||
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.
See Documentation/00-INDEX for a list of what is contained in each file.
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.