From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 57187 invoked by alias); 11 May 2018 11:16:36 -0000 Mailing-List: contact libc-alpha-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: libc-alpha-owner@sourceware.org Received: (qmail 57172 invoked by uid 89); 11 May 2018 11:16:35 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-1.9 required=5.0 tests=AWL,BAYES_00,KAM_NUMSUBJECT,RCVD_IN_DNSWL_NONE,SPF_HELO_PASS,SPF_PASS autolearn=no version=3.3.2 spammy=hacks X-HELO: EUR02-VE1-obe.outbound.protection.outlook.com Authentication-Results: spf=none (sender IP is ) smtp.mailfrom=Szabolcs.Nagy@arm.com; Cc: nd@arm.com Subject: Re: [Patch] Use VDSO interface for gettimeofday on aarch64 To: sellcey@cavium.com, libc-alpha References: <1525975253.28825.227.camel@cavium.com> From: Szabolcs Nagy Message-ID: <0b2d054c-d016-d825-780a-27e2b7c75c6a@arm.com> 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1;AM6PR08MB3285;7:I81FqtwYwMEALBH89szf2mF/8tA2Gdr7YowRaVucptEzcjLJA2Ekg6AjfK/FSLyV4CFTZg/m1wN7Fo9+6I4lL7X94kCa3g+8YYKMq8v0ijwZ4cCZyDX3JYHgxWQkF5GXWFa2uTOV7FzOCBGSFz+i49IyBSrhapqAt9Usoo8xYol3AfhUNJpcuSc92O88mZ6HjNst6dBsuwmvOzwWh0kCg+uujvO1dhM6uQUfwY6pfQDE+D1czQbQFcGdUG7XHXlR X-MS-Office365-Filtering-Correlation-Id: 541fac3a-0190-494e-667c-08d5b730a53a X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 11 May 2018 11:16:29.9611 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 541fac3a-0190-494e-667c-08d5b730a53a X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-Transport-CrossTenantHeadersStamped: AM6PR08MB3285 X-SW-Source: 2018-05/txt/msg00496.txt.bz2 On 10/05/18 19:00, Steve Ellcey wrote: > > This is a Aarch64 version of gettimeofday that uses the VDSO interface > when it is available.  I did a test with 100000000 gettimeofday calls > on a T88 and the time went from 7.1 seconds to 5.5 seconds.   I also > ran the glibc testsuite and I did not get any regressions. > > OK to checkin? > > Steve Ellcey > sellcey@cavium.com > > > 2018-05-10  Steve Ellcey   > > * sysdeps/unix/sysv/linux/aarch64/gettimeofday.c: New file. thanks, it looks reasonable approach, but the commit message should be fixed to indicate that this is a new VDSO mechanism (using ifunc) and why the old mechanism is still needed. please test with LD_BIND_NOW=1 too (this applies whenever ifuncs are involved, since they may behave differently when resolved lazily vs at load time and i don't see such test in glibc currently, a simple helloworld.c with gettimeofday usage is enough i think, it would be even better to add something like that to the test system) > + > +#ifdef SHARED > + note that static linked binaries do a real syscall now, this should be solved since users who really care about performance want to use static linked binaries, this is https://sourceware.org/bugzilla/show_bug.cgi?id=19767 (i think if !SHARED then global __vdso pointers can be initialized while the process is still single threaded using custom elf symbol lookup code and then current VSYSCALL mechanism should work) > +# define __gettimeofday __redirect___gettimeofday > +# include > +# undef __gettimeofday is this necessary? can we write out the declarations here? such macro redirection looks fragile to me. > +# define HAVE_VSYSCALL > +# include > +# include > + > +static int > +__gettimeofday_syscall (struct timeval *tv, struct timezone *tz) > +{ > +  return INLINE_VSYSCALL (gettimeofday, 2, tv, tz); > +} > + i'd call it __gettimeofday_vsyscall if you use VSYSCALL. is there a way _dl_vdso_vsym fails in the ifunc resolver but succeeds in VDSO_SETUP during _init? are there cases when __gettimeofday_syscall is called directly instead of via ifunc dispatch? (e.g. libc internal calls) vdso mechanisms are getting confusing, adding new mechanism is ok, but then either old ones should be cleaned up or comments added there clarifying which mechanism is used when (so the questions above are easy to answer). > +/* PREPARE_VERSION will need an __LP64__ ifdef when ILP32 support > +   goes in.  See _libc_vdso_platform_setup in > +   sysdeps/unix/sysv/linux/aarch64/init-first.c.  */ > + > +# undef INIT_ARCH > +# define INIT_ARCH() \ > +    PREPARE_VERSION (linux_version, "LINUX_2.6.39", 123718537); \ > +    void *vdso_gettimeofday = \ > +      _dl_vdso_vsym ("__kernel_gettimeofday", &linux_version); > + > +libc_ifunc_hidden (__redirect___gettimeofday, __gettimeofday, > +                    vdso_gettimeofday ?: (void *) __gettimeofday_syscall) > + this may do a vdso symbol look up whenever a dso is loaded that references gettimeofday (or when it's called in case of lazy binding) we could do the lookup only once at early init and use that in the ifunc resolver, but currently VDSO_SETUP runs after libc.so is relocated so i don't have a better idea. note that clock_gettime could use the same mechanism on aarch64 if we introduced a new abi symbol: __clock_gettime_noerrno and the public time.h had something like #define clock_gettime(id,ts) \ ( __id <= 6U \ ? __clock_gettime_noerrno (__id, __ts) \ : clock_gettime (__id, __ts) ) there might be better ways, not sure if glibc is happy with such hacks in public headers, but it's worth considering if you see significant performance difference. > +# undef libc_hidden_def > +# define libc_hidden_def(name)                               \ > +  __hidden_ver1 (__gettimeofday_syscall, __GI___gettimeofday,  \ > +               __gettimeofday_syscall); i'd use a new macro with different name here, e.g. #define hidden_vsyscall(name) __hidden_ver1 (name##_syscall,...) (or just write out explicitly what you want for SHARED vs !SHARED case separately.) does this mean internally in libc.so gettimeofday uses the existing VSYSCALL mechanism, but e.g. another dso like libpthread.so goes via ifunc? > + > +#else > + > +# include > +# include > +int > +__gettimeofday (struct timeval *tv, struct timezone *tz) > +{ > +  return INLINE_SYSCALL (gettimeofday, 2, tv, tz); > +} > +#endif > + > +libc_hidden_def (__gettimeofday) > +weak_alias (__gettimeofday, gettimeofday) > +libc_hidden_weak (gettimeofday) >