From nobody Sat Jan 11 22:42:18 2025 X-Original-To: freebsd-hackers@mlmmj.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mlmmj.nyi.freebsd.org (Postfix) with ESMTP id 4YVtnc5j2Pz5jwPG for ; Sat, 11 Jan 2025 22:42:28 +0000 (UTC) (envelope-from junchoon@dec.sakura.ne.jp) Received: from www121.sakura.ne.jp (www121.sakura.ne.jp [153.125.133.21]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 4YVtnc1TVjz4VTv; Sat, 11 Jan 2025 22:42:27 +0000 (UTC) (envelope-from junchoon@dec.sakura.ne.jp) Authentication-Results: mx1.freebsd.org; none Received: from kalamity.joker.local (124-18-43-234.area1a.commufa.jp [124.18.43.234]) (authenticated bits=0) by www121.sakura.ne.jp (8.17.1/8.17.1/[SAKURA-WEB]/20201212) with ESMTPA id 50BMgI2i030817; Sun, 12 Jan 2025 07:42:20 +0900 (JST) (envelope-from junchoon@dec.sakura.ne.jp) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=dec.sakura.ne.jp; s=s2405; t=1736635340; bh=8Gi5bMjcYT44EsdsiCKWmkmMNZaNipYEvQZazUDvASc=; h=Date:From:To:Cc:Subject:In-Reply-To:References; b=agMM/fiVYyfMuYv9IIS7MMidoOQTNLevwspWIzqGob+dZoGg3Q2m9DuOHqQpo3+MV iH0ZKw5UWUx7FFzwZuvP/+zP4FoK8xcxO3f3cvlcSihABOgerYm32shdHqY77dcCao ejVZXc7iOmDq98dklka8K6/w6lnB5PEyqJmNaoTU= Date: Sun, 12 Jan 2025 07:42:18 +0900 From: Tomoaki AOKI To: Warner Losh Cc: Mark Johnston , Konstantin Belousov , FreeBSD Hackers Subject: Re: widening ticks Message-Id: <20250112074218.baa88390293ead0e2836ec08@dec.sakura.ne.jp> In-Reply-To: References: <20250111131106.4d2657de20eeed7eef5c0b15@dec.sakura.ne.jp> <20250112043543.86b303419f954b2b287d39d1@dec.sakura.ne.jp> Organization: Junchoon corps X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.33; amd64-portbld-freebsd14.2) List-Id: Technical discussions relating to FreeBSD List-Archive: https://lists.freebsd.org/archives/freebsd-hackers List-Help: List-Post: List-Subscribe: List-Unsubscribe: Sender: owner-freebsd-hackers@FreeBSD.org Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 4YVtnc1TVjz4VTv X-Spamd-Bar: ---- X-Rspamd-Pre-Result: action=no action; module=replies; Message is reply to one we originated X-Spamd-Result: default: False [-4.00 / 15.00]; REPLY(-4.00)[]; ASN(0.00)[asn:7684, ipnet:153.125.128.0/18, country:JP] On Sat, 11 Jan 2025 13:12:24 -0700 Warner Losh wrote: > Why not have jiffiesjust be an alias for tickl at the assembler level, then > just have extern unsigned long jiffies; so the types match and we don't > have fragile macros? At the assembler level, long and unsigned long are the > sane for object definition. Basically I like this idea, but why I didn't proposed this is that "jiffies" is stated (sorry, not read the LinuxKPI code) to be unsigned long, while tickl is defined as (signed) int in the diff committed. Yes, I know it's just how the bit pattern of the same size is interpreted, but would need some safeguards in C part. > > Warner > > On Sat, Jan 11, 2025, 12:36 PM Tomoaki AOKI > wrote: > > > On Sat, 11 Jan 2025 11:34:06 -0500 > > Mark Johnston wrote: > > > > > On Sat, Jan 11, 2025 at 01:11:06PM +0900, Tomoaki AOKI wrote: > > > > On Wed, 8 Jan 2025 18:07:47 -0500 > > > > Mark Johnston wrote: > > > > > > > > > On Thu, Jan 09, 2025 at 12:18:48AM +0200, Konstantin Belousov wrote: > > > > > > On Wed, Jan 08, 2025 at 04:31:16PM -0500, Mark Johnston wrote: > > > > > > > The global "ticks" variable counts hardclock ticks, it's widely > > used in > > > > > > > the kernel for low-precision timekeeping. The linuxkpi provides > > a very > > > > > > > similar variable, "jiffies", but there's an incompatibility: the > > former > > > > > > > is a signed int and the latter is an unsigned long. It's not > > > > > > > particularly easy to paper over this difference, which has been > > > > > > > responsible for some nasty bugs, and modifying drivers to store > > the > > > > > > > jiffies value in a signed int is error-prone and a maintenance > > burden > > > > > > > that the linuxkpi is supposed to avoid. > > > > > > > > > > > > > > It would be nice to provide a compatible implementation of > > jiffies. I > > > > > > > can see a few approaches: > > > > > > > - Define a 64-bit ticks variable, say ticks64, and make > > hardclock() > > > > > > > update both ticks and ticks64. Then #define jiffies ticks64 > > on 64-bit > > > > > > > platforms. This is the simplest to implement, but it adds > > extra work > > > > > > > to hardclock() and is somewhat ugly. > > > > > > > - Make ticks an int64_t or a long and convert our native code > > > > > > > accordingly. This is cleaner but requires a lot of auditing > > to avoid > > > > > > > introducing bugs, though perhaps some code could be left > > unmodified, > > > > > > > implicitly truncating the value to an int. For example I think > > > > > > > sched_pctcpu_update() is fine. I've gotten an amd64 kernel to > > compile > > > > > > > and boot with this change, but it's hard to be confident in > > it. This > > > > > > > approach also has the potential downside of bloating > > structures that > > > > > > > store a ticks value, and it can't be MFCed. > > > > > > > - Introduce a 64-bit ticks variable, ticks64, and > > > > > > > #define ticks ((int)ticks64). This requires renaming any > > struct > > > > > > > fields and local vars named "ticks", of which there's a decent > > number, > > > > > > > but that can be done fairly mechanically. > > > > > > > > > > > > > > Is there another solution which avoids these pitfalls? If not, > > should > > > > > > > we go ahead with one of these approaches? If so, which one? > > > > > > > > > > > > You cannot do this in C, but can in asm: > > > > > > .data > > > > > > .globl ticksl, ticks > > > > > > .type ticksl, @object > > > > > > .type ticks, @object > > > > > > ticksl: .quad > > > > > > .size ticksl, 8 > > > > > > ticks =ticksl /* for little-endian */ > > > > > > /* ticks =ticksl + 4 for big-endian */ > > > > > > .size ticks, 4 > > > > > > > > > > > > > > > > > > Then update only ticksl in the hardclock(). > > > > > > > > > > I implemented your suggestion here: > > https://reviews.freebsd.org/D48383 > > > > > > > > As this is already committed to main, commenting here instead of review > > > > D48383. > > > > > > > > Maybe I'm too paranoid and overlooking something, but... > > > > > > > > *If "jiffies" in LinuxKPI is really unsigned, isn't there any > > > > possibilities that relies on its value to be larger than > > > > 0x7fffffffffffffff as a threshold? > > > > (Yes, it should be silly and non-realistic, but theoretically > > > > possible.) > > > > > > Ideally we would have > > > > > > #define jiffies ((unsigned long)ticksl) > > > > > > in the linuxkpi, but some Linux code uses "jiffies" as a struct field or > > > local variable name, so this doesn't quite work. > > > > > > In practice, the value is usually assigned to an unsigned long or used > > > as an operand where it would be implicitly promoted to an unsigned type, > > > so we don't see any incompatibilities. > > > > > > When jiffies is an int, code like the following can misbehave: > > > > > > unsigned long remain, timeout = jiffies + const; > > > ... > > > remain = timeout - jiffies; > > > if ((long)remain < 0) > > > /* timed out */ > > > > > > If (int)timeout and jiffies have different signs, as might happen close > > > to a rollover, the comparison won't work as expected. > > > > > > Linux has some macros (time_after() etc.) which are supposed to be used > > > instead of direct comparisons, but they're not always used. > > > > So ticksl should better be unsigned long if there's no reason to keep > > it signed, isn't it? > > > > > > > > *Is anywhere checking carry (sign) bit for int on LP32? > > > > Maybe it would be the reason if "jiffies" in LinuxKPI is really > > > > unsigned. > > > > > > Could you provide an example of what you mean? > > > > Not an example of code, but for example, when ticksl is at > > 0x7fffffffffffffff (positive value), ticks shoule be 0xffffffff > > (negative value), if I read the diff correctly. > > The same thing starts happening ticksl is at 0x0000000080000000 throug > > 0x00000000ffffffff and values alike. So signs (carry bits, usually the > > leftmost bit of each) should be checked separately for ticksl and ticks. > > > > Am I (hopefully) overlooking something? > > > > -- > > Tomoaki AOKI -- Tomoaki AOKI