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Tue, 16 Feb 2021 18:53:09 +0000 From: Nadav Amit To: Peter Zijlstra CC: Thomas Gleixner , "linux-kernel@vger.kernel.org" , Andy Lutomirski , Dave Hansen , Rik van Riel , Josh Poimboeuf Subject: Re: [PATCH v5 1/8] smp: Run functions concurrently in smp_call_function_many_cond() Thread-Topic: [PATCH v5 1/8] smp: Run functions concurrently in smp_call_function_many_cond() Thread-Index: AQHXBIFqKsC+lRA3wUC6VAD48B2JFqpbIOyA Date: Tue, 16 Feb 2021 18:53:09 +0000 Message-ID: <457D8FBC-8F64-48E9-B9E2-1A316DB0C2B6@vmware.com> References: <20210209221653.614098-1-namit@vmware.com> <20210209221653.614098-2-namit@vmware.com> In-Reply-To: Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: authentication-results: infradead.org; dkim=none (message not signed) header.d=none;infradead.org; dmarc=none action=none header.from=vmware.com; x-originating-ip: [24.6.216.183] x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: 94bb6361-0d0f-46b0-24cd-08d8d2ac1ab5 x-ms-traffictypediagnostic: BY3PR05MB8083: x-microsoft-antispam-prvs: x-ms-oob-tlc-oobclassifiers: OLM:6430; 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charset="us-ascii" Content-ID: Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-OriginatorOrg: vmware.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-AuthSource: BYAPR05MB4776.namprd05.prod.outlook.com X-MS-Exchange-CrossTenant-Network-Message-Id: 94bb6361-0d0f-46b0-24cd-08d8d2ac1ab5 X-MS-Exchange-CrossTenant-originalarrivaltime: 16 Feb 2021 18:53:09.5939 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: b39138ca-3cee-4b4a-a4d6-cd83d9dd62f0 X-MS-Exchange-CrossTenant-mailboxtype: HOSTED X-MS-Exchange-CrossTenant-userprincipalname: x/wsIKrVNX+vb0Cu3SZqbjpkFmHKj935p10TR/qso1e113ZOXXryFYv5s4hNBPuh5scnYLsXYPrhe8Xf93J11A== X-MS-Exchange-Transport-CrossTenantHeadersStamped: BY3PR05MB8083 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > On Feb 16, 2021, at 8:32 AM, Peter Zijlstra wrote: >=20 > On Tue, Feb 09, 2021 at 02:16:46PM -0800, Nadav Amit wrote: >> From: Nadav Amit >>=20 >> Currently, on_each_cpu() and similar functions do not exploit the >> potential of concurrency: the function is first executed remotely and >> only then it is executed locally. Functions such as TLB flush can take >> considerable time, so this provides an opportunity for performance >> optimization. >>=20 >> To do so, modify smp_call_function_many_cond(), to allows the callers to >> provide a function that should be executed (remotely/locally), and run >> them concurrently. Keep other smp_call_function_many() semantic as it is >> today for backward compatibility: the called function is not executed in >> this case locally. >>=20 >> smp_call_function_many_cond() does not use the optimized version for a >> single remote target that smp_call_function_single() implements. For >> synchronous function call, smp_call_function_single() keeps a >> call_single_data (which is used for synchronization) on the stack. >> Interestingly, it seems that not using this optimization provides >> greater performance improvements (greater speedup with a single remote >> target than with multiple ones). Presumably, holding data structures >> that are intended for synchronization on the stack can introduce >> overheads due to TLB misses and false-sharing when the stack is used for >> other purposes. >>=20 >> Reviewed-by: Dave Hansen >> Cc: Peter Zijlstra >> Cc: Rik van Riel >> Cc: Thomas Gleixner >> Cc: Andy Lutomirski >> Cc: Josh Poimboeuf >> Signed-off-by: Nadav Amit >=20 > Kernel-CI is giving me a regression that's most likely this patch: >=20 > https://nam04.safelinks.protection.outlook.com/?url=3Dhttps%3A%2F%2Fkern= elci.org%2Ftest%2Fcase%2Fid%2F602bdd621c979f83faaddcc6%2F&data=3D04%7C0= 1%7Cnamit%40vmware.com%7C7dc93f3b74d8488de06f08d8d2988b0a%7Cb39138ca3cee4b4= aa4d6cd83d9dd62f0%7C0%7C0%7C637490899907612612%7CUnknown%7CTWFpbGZsb3d8eyJW= IjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&s= data=3DPFs0ydMLh6xVfAQzAxSNd108YjxKMopNwxqsm82lEog%3D&reserved=3D0 >=20 > I'm not sure I can explain it yet. It did get me looking at > on_each_cpu() and it appears that wants to be converted too, something > like the below perhaps. Looks like a good cleanup, but I cannot say I understand the problem and ho= w it would solve it. Err...=