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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Thu, 16 May 2019 19:22:22 +0100 Received: from b06wcsmtp001.portsmouth.uk.ibm.com (b06wcsmtp001.portsmouth.uk.ibm.com [9.149.105.160]) by b06cxnps4076.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x4GIMLtc50004046 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 16 May 2019 18:22:21 GMT Received: from b06wcsmtp001.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 727C5A405C; Thu, 16 May 2019 18:22:21 +0000 (GMT) Received: from b06wcsmtp001.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id CFF65A405B; Thu, 16 May 2019 18:22:20 +0000 (GMT) Received: from localhost (unknown [9.102.3.1]) by b06wcsmtp001.portsmouth.uk.ibm.com (Postfix) with ESMTP; Thu, 16 May 2019 18:22:20 +0000 (GMT) Date: Thu, 16 May 2019 23:52:19 +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> <1557821918.xbleq18bk2.naveen@linux.ibm.com> <1557985279.4o349j5g2i.astroid@bobo.none> In-Reply-To: <1557985279.4o349j5g2i.astroid@bobo.none> 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: 19051618-0008-0000-0000-000002E76F13 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19051618-0009-0000-0000-00002254157A Message-Id: <1557989161.cjlaryiij4.naveen@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:, , definitions=2019-05-16_14:, , 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=432 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1905160115 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" Nicholas Piggin wrote: > Naveen N. Rao's on May 14, 2019 6:32 pm: >> 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 leave= s >>>>>> the mflr. mflr is executed by the branch unit that can only execute = one >>>>>> 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 d= o >>>>>> 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 m= flr >>>>> 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? >>=20 >> 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' >=20 > Good idea. Well the mflr r0 is harmless, so you can leave that in. > You just need to ensure it's not removed before the bl is. So nop > the bl _mcount, then synchronize_rcu_tasks(), then nop the mflr? The problem actually seems to be when we try to patch in the branch to=20 _mcount(), rather than when we are patching in the nop instructions=20 (i.e., the race is when we try to enable the function tracer, rather=20 than while disabling it). When we disable ftrace, we only need to ensure we patch out the branch=20 to _mcount() before patching out the preceding 'mflr r0'. I don't think=20 we need a synchronize_rcu_tasks() in that case. While enabling ftrace, we will first need to patch the preceding 'mflr=20 r0' (which would now be a 'nop') with 'b +8', then use=20 synchronize_rcu_tasks() and finally patch in 'bl _mcount()' followed by=20 'mflr r0'. I think that's what you meant, just that my reference to patching 'mflr=20 r0' with a 'b +8' should have called out that the mflr would have been=20 nop'ed out. - Naveen =