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Thu, 09 May 2024 00:46:37 -0700 (PDT) X-Google-Smtp-Source: AGHT+IEmSvzdCw9iXsAde+CQex6xKhWRl8ljB1dEZSk63p1UnNhtNjgqw+WKSP/6N6GSQe3WVMFozA== X-Received: by 2002:a05:600c:1c0a:b0:418:9941:ca28 with SMTP id 5b1f17b1804b1-41f719d62b4mr34878105e9.2.1715240797172; Thu, 09 May 2024 00:46:37 -0700 (PDT) Received: from gerbillo.redhat.com ([2a0d:3344:1b68:1b10:ff61:41fd:2ae4:da3a]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3502bbbc082sm944823f8f.107.2024.05.09.00.46.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 09 May 2024 00:46:36 -0700 (PDT) Message-ID: <6891de8fbc542340d157634ff1fe6701443995a5.camel@redhat.com> Subject: Re: [PATCH RFC net-next] net: cache the __dev_alloc_name() From: Paolo Abeni To: William Tu , netdev@vger.kernel.org Cc: jiri@nvidia.com, bodong@nvidia.com, kuba@kernel.org Date: Thu, 09 May 2024 09:46:35 +0200 In-Reply-To: <103033d0-f6e2-49ee-a8e2-ba23c6e9a6a1@nvidia.com> References: <20240506203207.1307971-1-witu@nvidia.com> <03c25d8e994e4388cb8bfd726ba738eea3c4dcdf.camel@redhat.com> <103033d0-f6e2-49ee-a8e2-ba23c6e9a6a1@nvidia.com> Autocrypt: addr=pabeni@redhat.com; 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charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.50.4 (3.50.4-1.fc39) Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Tue, 2024-05-07 at 11:55 -0700, William Tu wrote: >=20 > On 5/7/24 12:26 AM, Paolo Abeni wrote: > > External email: Use caution opening links or attachments > >=20 > >=20 > > On Mon, 2024-05-06 at 20:32 +0000, William Tu wrote: > > > When a system has around 1000 netdevs, adding the 1001st device becom= es > > > very slow. The devlink command to create an SF > > > $ devlink port add pci/0000:03:00.0 flavour pcisf \ > > > pfnum 0 sfnum 1001 > > > takes around 5 seconds, and Linux perf and flamegraph show 19% of tim= e > > > spent on __dev_alloc_name() [1]. > > >=20 > > > The reason is that devlink first requests for next available "eth%d". > > > And __dev_alloc_name will scan all existing netdev to match on "ethN"= , > > > set N to a 'inuse' bitmap, and find/return next available number, > > > in our case eth0. > > >=20 > > > And later on based on udev rule, we renamed it from eth0 to > > > "en3f0pf0sf1001" and with altname below > > > 14: en3f0pf0sf1001: ... > > > altname enp3s0f0npf0sf1001 > > >=20 > > > So eth0 is actually never being used, but as we have 1k "en3f0pf0sfN" > > > devices + 1k altnames, the __dev_alloc_name spends lots of time goint > > > through all existing netdev and try to build the 'inuse' bitmap of > > > pattern 'eth%d'. And the bitmap barely has any bit set, and it rescan= es > > > every time. > > >=20 > > > I want to see if it makes sense to save/cache the result, or is there > > > any way to not go through the 'eth%d' pattern search. The RFC patch > > > adds name_pat (name pattern) hlist and saves the 'inuse' bitmap. It s= aves > > > pattens, ex: "eth%d", "veth%d", with the bitmap, and lookup before > > > scanning all existing netdevs. > > An alternative heuristic that should be cheap and possibly reasonable > > could be optimistically check for 0.. > > availability, possibly restricting such attempt at scenarios where the > > total number of hashed netdevice names is somewhat high. > >=20 > > WDYT? > >=20 > > Cheers, > >=20 > > Paolo > Hi Paolo, >=20 > Thanks for your suggestion! > I'm not clear with that idea. >=20 > The current code has to do a full scan of all netdevs in a list, and the= =20 > name list is not sorted / ordered. So to get to know, ex: eth0 .. eth10,= =20 > we still need to do a full scan, find netdev with prefix "eth", and get= =20 > net available bit 11 (10+1). > And in another use case where users doesn't install UDEV rule to rename,= =20 > the system can actually create eth998, eth999, eth1000.... >=20 > What if we create prefix map (maybe using xarray) > idx=C2=A0=C2=A0 entry=3D(prefix, bitmap) > -------------------- > 0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 eth, 1111000000... > 1=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 veth, 1000000... > 2=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 can, 11100000... > 3=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 firewire, 00000... >=20 > but then we need to unset the bit when device is removed. > William Sorry for the late reply. I mean something alike the following (completely untested!!!): --- diff --git a/net/core/dev.c b/net/core/dev.c index d2ce91a334c1..0d428825f88a 100644 --- a/net/core/dev.c +++ b/net/core/dev.c @@ -1109,6 +1109,12 @@ static int __dev_alloc_name(struct net *net, const c= har *name, char *res) if (!p || p[1] !=3D 'd' || strchr(p + 2, '%')) return -EINVAL; =20 + for (i =3D 0; i < 4; ++i) { + snprintf(buf, IFNAMSIZ, name, i); + if (!__dev_get_by_name(net, buf)) + goto found; + } + /* Use one page as a bit array of possible slots */ inuse =3D bitmap_zalloc(max_netdevices, GFP_ATOMIC); if (!inuse) @@ -1144,6 +1150,7 @@ static int __dev_alloc_name(struct net *net, const ch= ar *name, char *res) if (i =3D=3D max_netdevices) return -ENFILE; =20 +found: /* 'res' and 'name' could overlap, use 'buf' as an intermediate buffer */ strscpy(buf, name, IFNAMSIZ); snprintf(res, IFNAMSIZ, buf, i); --- plus eventually some additional check to use such heuristic only if the total number of devices is significantly high. That would need some additional book-keeping, not added here. Cheers, Paolo