Fork of reMarkable kernel https://github.com/reMarkable/linux
dd0c41f8a7
This patch allows the user to disable write combined mapping
of the efifb framebuffer console using an nowc option.
A customer noticed major slowdowns while logging to the console
with write combining enabled, on other tasks running on the same
CPU. (10x or greater slow down on all other cores on the same CPU
as is doing the logging).
I reproduced this on a machine with dual CPUs.
Intel(R) Xeon(R) CPU E5-2609 v3 @ 1.90GHz (6 core)
I wrote a test that just mmaps the pci bar and writes to it in
a loop, while this was running in the background one a single
core with (taskset -c 1), building a kernel up to init/version.o
(taskset -c 8) went from 13s to 133s or so. I've yet to explain
why this occurs or what is going wrong I haven't managed to find
a perf command that in any way gives insight into this.
11,885,070,715 instructions # 1.39 insns per cycle
vs
12,082,592,342 instructions # 0.13 insns per cycle
is the only thing I've spotted of interest, I've tried at least:
dTLB-stores,dTLB-store-misses,L1-dcache-stores,LLC-store,LLC-store-misses,LLC-load-misses,LLC-loads,\mem-loads,mem-stores,iTLB-loads,iTLB-load-misses,cache-references,cache-misses
For now it seems at least a good idea to allow a user to disable write
combining if they see this until we can figure it out.
Note also most users get a real framebuffer driver loaded when kms
kicks in, it just happens on these machines the kernel didn't support
the gpu specific driver.
Signed-off-by: Dave Airlie <airlied@redhat.com>
Acked-by: Peter Jones <pjones@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
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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.