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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Tue, 14 May 2019 09:32:41 +0100 Received: from d06av24.portsmouth.uk.ibm.com (d06av24.portsmouth.uk.ibm.com [9.149.105.60]) by b06cxnps4075.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x4E8Wep745875332 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 14 May 2019 08:32:40 GMT Received: from d06av24.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 12A694203F; Tue, 14 May 2019 08:32:40 +0000 (GMT) Received: from d06av24.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id B6E5342047; Tue, 14 May 2019 08:32:39 +0000 (GMT) Received: from localhost (unknown [9.124.35.142]) by d06av24.portsmouth.uk.ibm.com (Postfix) with ESMTP; Tue, 14 May 2019 08:32:39 +0000 (GMT) Date: Tue, 14 May 2019 14:02:37 +0530 From: "Naveen N. Rao" Subject: Re: [RFC PATCH] powerpc/64/ftrace: mprofile-kernel patch out mflr To: linuxppc-dev@lists.ozlabs.org, Michael Ellerman , Nicholas Piggin References: <20190413015940.31170-1-npiggin@gmail.com> <871s13ujcf.fsf@concordia.ellerman.id.au> <1557729790.fw18xf9mdt.naveen@linux.ibm.com> <87tvdytwo0.fsf@concordia.ellerman.id.au> In-Reply-To: <87tvdytwo0.fsf@concordia.ellerman.id.au> User-Agent: astroid/0.14.0 (https://github.com/astroidmail/astroid) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable X-TM-AS-GCONF: 00 x-cbid: 19051408-0012-0000-0000-0000031B72D7 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19051408-0013-0000-0000-0000215409A3 Message-Id: <1557821918.xbleq18bk2.naveen@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:, , definitions=2019-05-14_05:, , signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=458 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1905140062 X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" Michael Ellerman wrote: > "Naveen N. Rao" writes: >> Michael Ellerman wrote: >>> Nicholas Piggin writes: >>>> The new mprofile-kernel mcount sequence is >>>> >>>> mflr r0 >>>> bl _mcount >>>> >>>> Dynamic ftrace patches the branch instruction with a noop, but leaves >>>> the mflr. mflr is executed by the branch unit that can only execute on= e >>>> per cycle on POWER9 and shared with branches, so it would be nice to >>>> avoid it where possible. >>>> >>>> This patch is a hacky proof of concept to nop out the mflr. Can we do >>>> this or are there races or other issues with it? >>>=20 >>> There's a race, isn't there? >>>=20 >>> We have a function foo which currently has tracing disabled, so the mfl= r >>> and bl are nop'ed out. >>>=20 >>> CPU 0 CPU 1 >>> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D >>> bl foo >>> nop (ie. not mflr) >>> -> interrupt >>> something else enable tracing for foo >>> ... patch mflr and branch >>> <- rfi >>> bl _mcount >>>=20 >>> So we end up in _mcount() but with r0 not populated. >> >> Good catch! Looks like we need to patch the mflr with a "b +8" similar=20 >> to what we do in __ftrace_make_nop(). >=20 > Would that actually make it any faster though? Nick? Ok, how about doing this as a 2-step process? 1. patch 'mflr r0' with a 'b +8' synchronize_rcu_tasks() 2. convert 'b +8' to a 'nop' - Naveen =