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Wed, 20 Nov 2024 18:10:06 GMT Received: from smtpav07.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 1081820043; Wed, 20 Nov 2024 18:10:06 +0000 (GMT) Received: from smtpav07.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 2205F20040; Wed, 20 Nov 2024 18:10:04 +0000 (GMT) Received: from [9.39.17.146] (unknown [9.39.17.146]) by smtpav07.fra02v.mail.ibm.com (Postfix) with ESMTP; Wed, 20 Nov 2024 18:10:03 +0000 (GMT) Message-ID: <5733e508-f503-46cd-8874-d0c82355ae11@linux.ibm.com> Date: Wed, 20 Nov 2024 23:40:03 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 2/2] powerpc: Large user copy aware of full:rt:lazy preemption To: Sebastian Andrzej Siewior Cc: mpe@ellerman.id.au, linuxppc-dev@lists.ozlabs.org, npiggin@gmail.com, christophe.leroy@csgroup.eu, maddy@linux.ibm.com, ankur.a.arora@oracle.com, linux-kernel@vger.kernel.org References: <20241116192306.88217-1-sshegde@linux.ibm.com> <20241116192306.88217-3-sshegde@linux.ibm.com> <20241120080041.aNFNqWCz@linutronix.de> From: Shrikanth Hegde Content-Language: en-US In-Reply-To: <20241120080041.aNFNqWCz@linutronix.de> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: IK3UphI6nllhhhVGFQWdEnBf_ObDnm6Y X-Proofpoint-GUID: y09NOR_0zNxj7KvIFErNtz_TijExyeub X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1051,Hydra:6.0.680,FMLib:17.12.62.30 definitions=2024-10-15_01,2024-10-11_01,2024-09-30_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 impostorscore=0 phishscore=0 mlxlogscore=999 lowpriorityscore=0 priorityscore=1501 suspectscore=0 malwarescore=0 clxscore=1015 mlxscore=0 spamscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.19.0-2409260000 definitions=main-2411200124 On 11/20/24 13:30, Sebastian Andrzej Siewior wrote: > On 2024-11-17 00:53:06 [+0530], Shrikanth Hegde wrote: >> Large user copy_to/from (more than 16 bytes) uses vmx instructions to >> speed things up. Once the copy is done, it makes sense to try schedule >> as soon as possible for preemptible kernels. So do this for >> preempt=full/lazy and rt kernel. >> >> Not checking for lazy bit here, since it could lead to unnecessary >> context switches. >> >> Suggested-by: Sebastian Andrzej Siewior >> Signed-off-by: Shrikanth Hegde >> --- >> arch/powerpc/lib/vmx-helper.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/arch/powerpc/lib/vmx-helper.c b/arch/powerpc/lib/vmx-helper.c >> index d491da8d1838..58ed6bd613a6 100644 >> --- a/arch/powerpc/lib/vmx-helper.c >> +++ b/arch/powerpc/lib/vmx-helper.c >> @@ -45,7 +45,7 @@ int exit_vmx_usercopy(void) >> * set and we are preemptible. The hack here is to schedule a >> * decrementer to fire here and reschedule for us if necessary. >> */ >> - if (IS_ENABLED(CONFIG_PREEMPT) && need_resched()) >> + if (IS_ENABLED(CONFIG_PREEMPTION) && need_resched()) >> set_dec(1); > > Now looking at this again there is a comment why preempt_enable() is > bad. An interrupt between preempt_enable_no_resched() and set_dec() is > fine because irq-exit would preempt properly? I think so. AFAIU the comment says issue lies with amr register not being saved across context switch. interrupt_exit_kernel_prepare saves it and restore using kuap_kernel_restore. Regular preemption works > again once copy_to_user() is done? So if you copy 1GiB, you are blocked > for that 1GiB? yes, regular preemption would work on exit of copy_to_user. Since the preempt_disable was done before copy starts, i think yes, it would be blocked until it is complete. > >> return 0; >> } > > Sebastian nick, mpe; please correct me if i am wrong.