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Peter Zijlstra aab8621776 lib/int_sqrt: optimize small argument 
commit 3f3295709e upstream.

The current int_sqrt() computation is sub-optimal for the case of small
@x.  Which is the interesting case when we're going to do cumulative
distribution functions on idle times, which we assume to be a random
variable, where the target residency of the deepest idle state gives an
upper bound on the variable (5e6ns on recent Intel chips).

In the case of small @x, the compute loop:

	while (m != 0) {
		b = y + m;
		y >>= 1;

		if (x >= b) {
			x -= b;
			y += m;
		}
		m >>= 2;
	}

can be reduced to:

	while (m > x)
		m >>= 2;

Because y==0, b==m and until x>=m y will remain 0.

And while this is computationally equivalent, it runs much faster
because there's less code, in particular less branches.

      cycles:                 branches:              branch-misses:

OLD:

hot:   45.109444 +- 0.044117  44.333392 +- 0.002254  0.018723 +- 0.000593
cold: 187.737379 +- 0.156678  44.333407 +- 0.002254  6.272844 +- 0.004305

PRE:

hot:   67.937492 +- 0.064124  66.999535 +- 0.000488  0.066720 +- 0.001113
cold: 232.004379 +- 0.332811  66.999527 +- 0.000488  6.914634 +- 0.006568

POST:

hot:   43.633557 +- 0.034373  45.333132 +- 0.002277  0.023529 +- 0.000681
cold: 207.438411 +- 0.125840  45.333132 +- 0.002277  6.976486 +- 0.004219

Averages computed over all values <128k using a LFSR to generate order.
Cold numbers have a LFSR based branch trace buffer 'confuser' ran between
each int_sqrt() invocation.

Link: http://lkml.kernel.org/r/20171020164644.876503355@infradead.org
Fixes: 30493cc9dd ("lib/int_sqrt.c: optimize square root algorithm")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Suggested-by: Anshul Garg <aksgarg1989@gmail.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Joe Perches <joe@perches.com>
Cc: David Miller <davem@davemloft.net>
Cc: Matthew Wilcox <mawilcox@microsoft.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Michael Davidson <md@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2019-03-27 14:13:54 +09:00
arch x86/unwind: Add hardcoded ORC entry for NULL 2019-03-27 14:13:54 +09:00
block blk-mq: fix a hung issue when fsync 2019-02-20 10:20:44 +01:00
certs Replace magic for trusting the secondary keyring with #define 2018-09-09 19:55:54 +02:00
crypto crypto: pcbc - remove bogus memcpy()s with src == dest 2019-03-23 14:35:21 +01:00
Documentation dt-bindings: eeprom: at24: add "atmel,24c2048" compatible string 2019-02-20 10:20:44 +01:00
drivers drm: Reorder set_property_atomic to avoid returning with an active ww_ctx 2019-03-27 14:13:54 +09:00
firmware
fs ext4: brelse all indirect buffer in ext4_ind_remove_space() 2019-03-27 14:13:53 +09:00
include libceph: wait for latest osdmap in ceph_monc_blacklist_add() 2019-03-27 14:13:51 +09:00
init Revert "mm: use early_pfn_to_nid in page_ext_init" 2019-03-23 14:35:13 +01:00
ipc ipc/sem.c: prevent queue.status tearing in semop 2018-09-05 09:26:30 +02:00
kernel locking/lockdep: Add debug_locks check in __lock_downgrade() 2019-03-27 14:13:54 +09:00
lib lib/int_sqrt: optimize small argument 2019-03-27 14:13:54 +09:00
mm mm/vmalloc: fix size check for remap_vmalloc_range_partial() 2019-03-23 14:35:24 +01:00
net netfilter: ebtables: remove BUGPRINT messages 2019-03-27 14:13:54 +09:00
samples samples: mei: use /dev/mei0 instead of /dev/mei 2019-02-15 08:09:12 +01:00
scripts scripts/gdb: fix lx-version string output 2019-02-12 19:46:10 +01:00
security security/selinux: fix SECURITY_LSM_NATIVE_LABELS on reused superblock 2019-03-23 14:35:26 +01:00
sound ALSA: hda - Enforces runtime_resume after S3 and S4 for each codec 2019-03-27 14:13:54 +09:00
tools objtool: Move objtool_file struct off the stack 2019-03-27 14:13:52 +09:00
usr
virt KVM: Call kvm_arch_memslots_updated() before updating memslots 2019-03-23 14:35:31 +01:00
.cocciconfig
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.gitignore kbuild: rpm-pkg: keep spec file until make mrproper 2018-02-13 10:19:46 +01:00
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MAINTAINERS MAINTAINERS: Add Sasha as a stable branch maintainer 2018-12-01 09:42:50 +01:00
Makefile Linux 4.14.108 2019-03-23 14:35:32 +01:00
README

README 

Linux kernel
============

This file was moved to Documentation/admin-guide/README.rst

Please notice that there are several guides for kernel developers and users.
These guides can be rendered in a number of formats, like HTML and PDF.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.

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.