Date: Sun, 22 Oct 2017 20:17:08 +0200 From: Hans Petter Selasky <hps@selasky.org> To: Mateusz Guzik <mjguzik@gmail.com> 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: <7f65c32c-4f7d-9424-f106-8b699d19f684@selasky.org> In-Reply-To: <CAGudoHGRynhU569ZHinE3hxvK2vYx947E=SJ4%2BLFjkCSJGPXGA@mail.gmail.com> References: <201710221342.v9MDguCC074682@repo.freebsd.org> <316ecd86-a508-fb36-e33c-ba32f5cb8073@selasky.org> <CAGudoHFcBmsncg5k2cc4CGCbX6mhNOmcFKTNjWMgWskVbYWzNQ@mail.gmail.com> <b4cf9de2-cafc-1502-7f63-50a04d68cafe@selasky.org> <CAGudoHGRynhU569ZHinE3hxvK2vYx947E=SJ4%2BLFjkCSJGPXGA@mail.gmail.com>
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On 10/22/17 19:37, Mateusz Guzik wrote: > 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. > Hi, If the linker supports this, that's the best. Else you'll need a custom tool. sysinit.c might give you some ideas how to implement it in a cross-OS way. --HPS
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