Date: Sun, 22 Oct 2017 19:37:46 +0200 From: Mateusz Guzik <mjguzik@gmail.com> To: Hans Petter Selasky <hps@selasky.org> Cc: Mateusz Guzik <mjg@freebsd.org>, "src-committers@freebsd.org" <src-committers@freebsd.org>, "svn-src-all@freebsd.org" <svn-src-all@freebsd.org>, "svn-src-head@freebsd.org" <svn-src-head@freebsd.org> Subject: Re: svn commit: r324863 - in head/sys: kern sys Message-ID: <CAGudoHGRynhU569ZHinE3hxvK2vYx947E=SJ4%2BLFjkCSJGPXGA@mail.gmail.com> In-Reply-To: <b4cf9de2-cafc-1502-7f63-50a04d68cafe@selasky.org> References: <201710221342.v9MDguCC074682@repo.freebsd.org> <316ecd86-a508-fb36-e33c-ba32f5cb8073@selasky.org> <CAGudoHFcBmsncg5k2cc4CGCbX6mhNOmcFKTNjWMgWskVbYWzNQ@mail.gmail.com> <b4cf9de2-cafc-1502-7f63-50a04d68cafe@selasky.org>
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On Sun, Oct 22, 2017 at 6:59 PM, Hans Petter Selasky <hps@selasky.org> wrote: > On 10/22/17 16:50, Mateusz Guzik wrote: > >> On Sun, Oct 22, 2017 at 04:24:41PM +0200, Hans Petter Selasky wrote: >> >>> On 10/22/17 15:42, Mateusz Guzik wrote: >>> >>>> Author: mjg >>>> Date: Sun Oct 22 13:42:56 2017 >>>> New Revision: 324863 >>>> URL: https://svnweb.freebsd.org/changeset/base/324863 >>>> >>>> Log: >>>> Change kdb_active type to u_char. >>>> Fixes warnings from gcc and keeps the small size. Perhaps nesting >>>> >>> should be moved >> >>> to another variablle. >>>> Reported by: ngie >>>> >>>> Modified: >>>> head/sys/kern/subr_kdb.c >>>> head/sys/sys/kdb.h >>>> >>>> Modified: head/sys/kern/subr_kdb.c >>>> >>>> ============================================================ >> ================== >> >>> --- head/sys/kern/subr_kdb.c Sun Oct 22 12:12:52 2017 (r324862) >>>> +++ head/sys/kern/subr_kdb.c Sun Oct 22 13:42:56 2017 (r324863) >>>> @@ -50,7 +50,7 @@ __FBSDID("$FreeBSD$"); >>>> #include <machine/smp.h> >>>> #endif >>>> -bool __read_frequently kdb_active = 0; >>>> +u_char __read_frequently kdb_active = 0; >>>> static void *kdb_jmpbufp = NULL; >>>> struct kdb_dbbe *kdb_dbbe = NULL; >>>> static struct pcb kdb_pcb; >>>> >>>> Modified: head/sys/sys/kdb.h >>>> >>>> ============================================================ >> ================== >> >>> --- head/sys/sys/kdb.h Sun Oct 22 12:12:52 2017 (r324862) >>>> +++ head/sys/sys/kdb.h Sun Oct 22 13:42:56 2017 (r324863) >>>> @@ -59,7 +59,7 @@ struct kdb_dbbe { >>>> }; \ >>>> DATA_SET(kdb_dbbe_set, name##_dbbe) >>>> -extern bool kdb_active; /* Non-zero while in debugger. */ >>>> +extern u_char kdb_active; /* Non-zero while in debugger. */ >>>> extern int debugger_on_panic; /* enter the debugger on panic. >>>> >>> */ >> >>> extern struct kdb_dbbe *kdb_dbbe; /* Default debugger backend or >>>> >>> NULL. */ >> >>> extern struct trapframe *kdb_frame; /* Frame to kdb_trap(). */ >>>> >>>> >>>> >>> Should we add __aligned(8) to this definition? >>> >>> ./systm.h:#define __read_frequently >>> >> __section(".data.read_frequently") >> >>> >>> It will prevent commonly read variables from residing in two different >>> cache-lines on x86 and amd64 at least??? >>> >>> >> I don't follow. This would *increase* alignemnt requirement and in >> particular prevent bool variables from being put in consecutive bytes. >> >> To answer the question from your other e-mail, the bigger the type the >> worse it is as it takes more space. The idea is to change all frequently >> read and effectively bool variables from int to bool so that more of >> them fit in one cacheline. >> >> Right now there is nothing to nicely sort them to get rid of holes, but >> I'm tinkering with automagic size addition to section name. >> >> > Hi, > > The point is that for x86 there is no alignment so the variables get > packed back to back, and then you sometimes get not so smart layouts, like > that an integer crosses a cache line. > > They are aligned to their size and this creates holes, like here: ffffffff8112873c D kdb_active ---------------- ffffffff81128740 D audit_enabled Regarding automation: Maybe the idea behind sysinit can be used: > sys/boot/usb/tools/sysinit.c > I don't know how this can be plugged here. Would this require defining variables elsewhere? Preferably they would be sorted by the linker. -- Mateusz Guzik <mjguzik gmail.com>
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