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If a task is scheduled out and receives a signal then it won't be able to take the fastpath because the registers aren't available. The slowpath is more expensive compared to XRSTOR + XSAVE which usually succeeds. Here are some clock_gettime() numbers from a bigger box with AVX512 during bootup: - __fpregs_load_activate() takes 140ns - 350ns. If it was the most recent FPU context on the CPU then the optimisation in __fpregs_load_activate() will skip the load (which was disabled during the test). - copy_fpregs_to_sigframe() takes 200ns - 450ns if it succeeds. On a pagefault it is 1.8us - 3us usually in the 2.6us area. - The slowpath takes 1.5us - 6us. Usually in the 2.6us area. My testcases (including lat_sig) take the fastpath without __fpregs_load_activate(). I expect this to be the majority. Since the slowpath is in the >1us area it makes sense to load the registers and attempt to save them directly. The direct save may fail but should only happen on the first invocation or after fork() while the page is read-only. [ bp: Massage a bit. ] Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Signed-off-by: Borislav Petkov <bp@suse.de> Reviewed-by: Dave Hansen <dave.hansen@intel.com> Reviewed-by: Thomas Gleixner <tglx@linutronix.de> Cc: Andy Lutomirski <luto@kernel.org> Cc: "H. Peter Anvin" <hpa@zytor.com> Cc: Ingo Molnar <mingo@redhat.com> Cc: Jann Horn <jannh@google.com> Cc: "Jason A. Donenfeld" <Jason@zx2c4.com> Cc: kvm ML <kvm@vger.kernel.org> Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: Radim Krčmář <rkrcmar@redhat.com> Cc: Rik van Riel <riel@surriel.com> Cc: x86-ml <x86@kernel.org> Link: https://lkml.kernel.org/r/20190403164156.19645-27-bigeasy@linutronix.de |
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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.